diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 37c7b50f..7ff6e8f3 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -3,14 +3,29 @@ name: Continuous Integration
 on: [push]
 
 jobs:
-  ubuntu-unit-test:
+  mypy:
+    name: mypy check
+    runs-on: ubuntu-18.04
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Python 3.7
+      uses: actions/setup-python@v2
+      with:
+        python-version: '3.7'
+    - name: Install mypy
+      run: |
+        cat scripts/requirements_dev.txt | grep "mypy" | xargs pip install
+    - name: Check
+      run: |
+        scripts/mypy.sh --version
+        scripts/mypy.sh
 
+  ubuntu-unit-test:
     runs-on: ubuntu-18.04
     strategy:
       matrix:
         mpi-vendor: ['openmpi', 'mpich']
         python-version: [3.7, 3.8]
-
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}
@@ -33,15 +48,13 @@ jobs:
         pip list
     - name: Unit Test
       run: make test_torch_basic test_torch_ops test_torch_hier_ops
-      
+
   macos-unit-test:
-    
     runs-on: macos-10.15
     strategy:
       matrix:
         mpi-vendor: ['openmpi', 'mpich']
         python-version: [3.7, 3.8]
-
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}
@@ -63,4 +76,4 @@ jobs:
         mpiexec --version
         pip list
     - name: Unit Test
-      run: OVERSUBSCRIBE=1 make test_torch_basic test_torch_hier_ops test_torch_ops test_torch_win_ops
\ No newline at end of file
+      run: OVERSUBSCRIBE=1 make test_torch_basic test_torch_hier_ops test_torch_ops test_torch_win_ops
diff --git a/bluefog/common/topology_util.py b/bluefog/common/topology_util.py
index e77411a4..138c8acb 100644
--- a/bluefog/common/topology_util.py
+++ b/bluefog/common/topology_util.py
@@ -116,11 +116,11 @@ def ExponentialGraph(size: int, base: int = 2) -> nx.DiGraph:
             x.append(1.0)
         else:
             x.append(0.0)
-    x = np.array(x)
-    x /= x.sum()
+    y = np.array(x)
+    y /= y.sum()
     topo = np.empty((size, size))
     for i in range(size):
-        topo[i] = np.roll(x, i)
+        topo[i] = np.roll(y, i)
     G = nx.from_numpy_array(topo, create_using=nx.DiGraph)
     return G
 
@@ -148,11 +148,11 @@ def SymmetricExponentialGraph(size: int, base: int = 4) -> nx.DiGraph:
             x.append(1.0)
         else:
             x.append(0.0)
-    x = np.array(x)
-    x /= x.sum()
+    y = np.array(x)
+    y /= y.sum()
     topo = np.empty((size, size))
     for i in range(size):
-        topo[i] = np.roll(x, i)
+        topo[i] = np.roll(y, i)
     G = nx.from_numpy_array(topo, create_using=nx.DiGraph)
     return G
 
diff --git a/bluefog/common/util.py b/bluefog/common/util.py
index f6c61aeb..59b84fdb 100644
--- a/bluefog/common/util.py
+++ b/bluefog/common/util.py
@@ -22,7 +22,7 @@
 EXTENSIONS = ['tensorflow', 'torch']
 
 def is_running_from_ipython():
-    from IPython import get_ipython
+    from IPython import get_ipython # type: ignore
     return get_ipython() is not None
 
 def get_ext_suffix():
diff --git a/bluefog/run/env_util.py b/bluefog/run/env_util.py
index 2b1bfced..8f8c9929 100644
--- a/bluefog/run/env_util.py
+++ b/bluefog/run/env_util.py
@@ -52,7 +52,7 @@ def is_open_mpi_installed():
 
 def is_ipyparallel_installed():
     try:
-        import ipyparallel  # pylint: disable=unused-import
+        import ipyparallel # type: ignore # pylint: disable=unused-import
         return True
     except ImportError:
         return False
diff --git a/bluefog/run/horovod_driver.py b/bluefog/run/horovod_driver.py
index 735a8d79..8b9a5cf5 100644
--- a/bluefog/run/horovod_driver.py
+++ b/bluefog/run/horovod_driver.py
@@ -18,7 +18,7 @@
 import os
 import sys
 
-import six
+import six # type: ignore
 from bluefog.run.horovodrun.common.util import codec, safe_shell_exec, timeout, secret
 from bluefog.run.horovodrun.driver import driver_service
 from bluefog.run.horovodrun.task import task_service
diff --git a/bluefog/run/horovodrun/common/util/codec.py b/bluefog/run/horovodrun/common/util/codec.py
index 4578255b..a4003f71 100644
--- a/bluefog/run/horovodrun/common/util/codec.py
+++ b/bluefog/run/horovodrun/common/util/codec.py
@@ -14,7 +14,7 @@
 # ==============================================================================
 
 import base64
-import cloudpickle
+import cloudpickle # type: ignore
 
 
 def loads_base64(encoded):
diff --git a/bluefog/run/horovodrun/common/util/network.py b/bluefog/run/horovodrun/common/util/network.py
index 1ad4d677..0b4a52a4 100644
--- a/bluefog/run/horovodrun/common/util/network.py
+++ b/bluefog/run/horovodrun/common/util/network.py
@@ -17,10 +17,10 @@
 import socket
 import struct
 import threading
-import cloudpickle
-import psutil
+import cloudpickle # type: ignore
+import psutil # type: ignore
 
-from six.moves import queue, socketserver
+from six.moves import queue, socketserver # type: ignore
 
 from bluefog.run.horovodrun.common.util import secret
 
diff --git a/bluefog/run/horovodrun/common/util/safe_shell_exec.py b/bluefog/run/horovodrun/common/util/safe_shell_exec.py
index 9ae2d2c5..1216e509 100644
--- a/bluefog/run/horovodrun/common/util/safe_shell_exec.py
+++ b/bluefog/run/horovodrun/common/util/safe_shell_exec.py
@@ -14,7 +14,7 @@
 # ==============================================================================
 
 import os
-import psutil
+import psutil # type: ignore
 import signal
 import subprocess
 import sys
diff --git a/bluefog/run/horovodrun/util/cache.py b/bluefog/run/horovodrun/util/cache.py
index dc83661f..5f81f303 100644
--- a/bluefog/run/horovodrun/util/cache.py
+++ b/bluefog/run/horovodrun/util/cache.py
@@ -17,7 +17,7 @@
 import errno
 import os
 import threading
-import cloudpickle
+import cloudpickle # type: ignore
 
 
 class Cache(object):
diff --git a/bluefog/run/horovodrun/util/network.py b/bluefog/run/horovodrun/util/network.py
index fbf2e5d9..d41b9958 100644
--- a/bluefog/run/horovodrun/util/network.py
+++ b/bluefog/run/horovodrun/util/network.py
@@ -1,5 +1,5 @@
 import socket
-import psutil
+import psutil # type: ignore
 
 from bluefog.run.horovodrun.util import threads
 
diff --git a/bluefog/run/horovodrun/util/threads.py b/bluefog/run/horovodrun/util/threads.py
index 78c10b36..5ba1bedd 100644
--- a/bluefog/run/horovodrun/util/threads.py
+++ b/bluefog/run/horovodrun/util/threads.py
@@ -15,7 +15,7 @@
 
 import threading
 
-from six.moves import queue
+from six.moves import queue # type: ignore
 
 
 def execute_function_multithreaded(fn,
diff --git a/bluefog/run/interactive_run.py b/bluefog/run/interactive_run.py
index f4273409..1a8f5335 100644
--- a/bluefog/run/interactive_run.py
+++ b/bluefog/run/interactive_run.py
@@ -22,7 +22,7 @@
 import time
 from typing import Dict, List
 
-import ipyparallel as ipp
+import ipyparallel as ipp # type: ignore
 import bluefog
 from bluefog.run import env_util, network_util, horovod_driver
 
@@ -318,7 +318,7 @@ def multiple_machines_launch(args, env: Dict[str, str],
         common_intfs = horovod_driver.driver_fn(all_host_names, local_host_names,
                                                 args.ssh_port, args.verbose)
     else:
-        common_intfs = [args.nic]
+        common_intfs = set(args.nic)
 
     tcp_intf_arg = '-mca btl_tcp_if_include {common_intfs}'.format(
         common_intfs=','.join(common_intfs)) if common_intfs else ''
@@ -384,8 +384,6 @@ def multiple_machines_launch(args, env: Dict[str, str],
                 ib_arg=ib_arg,
                 nccl_socket_intf_arg=nccl_socket_intf_arg,
                 extra_flags=extra_flags,
-                env=' '.join('-x %s' % key for key in env.keys()
-                             if env_util.is_exportable(key)),
                 command=ipengine_command)
     )
     p_engine = subprocess.Popen(mpi_ipengine_command, shell=True, env=env)
diff --git a/bluefog/run/network_util.py b/bluefog/run/network_util.py
index bcd3fbcb..46a7da65 100644
--- a/bluefog/run/network_util.py
+++ b/bluefog/run/network_util.py
@@ -20,7 +20,7 @@
 import re
 import socket
 
-import psutil
+import psutil # type: ignore
 from bluefog.run.horovodrun.common.util import safe_shell_exec
 
 # Number of retries for sshing into the hosts
diff --git a/bluefog/run/run.py b/bluefog/run/run.py
index c2bc80cc..2364ca1b 100644
--- a/bluefog/run/run.py
+++ b/bluefog/run/run.py
@@ -16,14 +16,8 @@
 
 import argparse
 import os
-import re
 import shlex
-import socket
-import subprocess
-import sys
-import traceback
 
-import psutil
 import bluefog
 from bluefog.run import env_util, network_util, horovod_driver
 
diff --git a/bluefog/tensorflow/__init__.py b/bluefog/tensorflow/__init__.py
index 072ecd8f..d9c6b8b9 100644
--- a/bluefog/tensorflow/__init__.py
+++ b/bluefog/tensorflow/__init__.py
@@ -4,7 +4,7 @@
 
 import collections
 import os
-import tensorflow as tf
+import tensorflow as tf # type: ignore
 
 from bluefog.common.util import check_extension
 check_extension('bluefog.tensorflow', __file__, 'mpi_lib')
diff --git a/bluefog/tensorflow/mpi_ops.py b/bluefog/tensorflow/mpi_ops.py
index 263d3256..98d31081 100644
--- a/bluefog/tensorflow/mpi_ops.py
+++ b/bluefog/tensorflow/mpi_ops.py
@@ -19,10 +19,10 @@
 from __future__ import print_function
 
 import re
-import tensorflow as tf
-from tensorflow.python.framework import load_library
-from tensorflow.python.framework import ops
-from tensorflow.python.platform import resource_loader
+import tensorflow as tf # type: ignore
+from tensorflow.python.framework import load_library # type: ignore
+from tensorflow.python.framework import ops # type: ignore
+from tensorflow.python.platform import resource_loader # type: ignore
 
 from bluefog.common.basics import BlueFogBasics
 from bluefog.common.util import get_ext_suffix
diff --git a/bluefog/tensorflow/optimizers.py b/bluefog/tensorflow/optimizers.py
index fbd155ad..10e7e502 100644
--- a/bluefog/tensorflow/optimizers.py
+++ b/bluefog/tensorflow/optimizers.py
@@ -14,7 +14,7 @@
 # limitations under the License.
 # ==============================================================================
 
-import tensorflow as tf
+import tensorflow as tf # type: ignore
 
 from bluefog.tensorflow.mpi_ops import allreduce, broadcast, size
 from bluefog.tensorflow.util import _executing_eagerly, _cache
diff --git a/bluefog/tensorflow/util.py b/bluefog/tensorflow/util.py
index e3b8f31c..1de817e5 100644
--- a/bluefog/tensorflow/util.py
+++ b/bluefog/tensorflow/util.py
@@ -16,11 +16,11 @@
 
 from distutils.version import LooseVersion
 
-import tensorflow
+import tensorflow # type: ignore
 
 # Eager Mode has been introduced in TF 1.7.0
 if LooseVersion(tensorflow.__version__) >= LooseVersion('1.7.0'):
-    from tensorflow.python.eager import context
+    from tensorflow.python.eager import context # type: ignore
     _has_eager = True
 else:
     _has_eager = False
diff --git a/bluefog/torch/optimizers.py b/bluefog/torch/optimizers.py
index 86b3e721..65bf0676 100644
--- a/bluefog/torch/optimizers.py
+++ b/bluefog/torch/optimizers.py
@@ -23,6 +23,7 @@
 import warnings
 
 import torch
+from torch.optim import Optimizer
 import bluefog.torch as bf
 
 class CommunicationType(Enum):
@@ -163,7 +164,7 @@ def _timeline_forward_end_hook(module, *unused):
             *pre_forward_hook_handles, *forward_end_hook_handles]
 
 
-class _DistributedOptimizer(torch.optim.Optimizer):
+class _DistributedOptimizer(Optimizer):
     def __init__(self, params, model, backward_passes_per_step=1):
         super(self.__class__, self).__init__(params)
 
@@ -294,7 +295,7 @@ def zero_grad(self):
         return super(self.__class__, self).zero_grad()
 
 
-class _DistributedReduceOptimizer(torch.optim.Optimizer):
+class _DistributedReduceOptimizer(Optimizer):
     """ A distributed optimizer wrapper over torch optimizer.
 
     Arguments:
@@ -482,7 +483,7 @@ def step(self, closure=None):
         return super(self.__class__, self).step(closure)
 
 
-class _DistributedAdaptThenCombineOptimizer(torch.optim.Optimizer):
+class _DistributedAdaptThenCombineOptimizer(Optimizer):
     def __init__(self, params, model, communication_type, backward_passes_per_step=1):
         super(self.__class__, self).__init__(params)
 
@@ -841,7 +842,7 @@ def zero_grad(self):
         return super(self.__class__, self).zero_grad()
 
 
-class _DistributedWinOptimizer(torch.optim.Optimizer):
+class _DistributedWinOptimizer(Optimizer):
 
     def __init__(self, params, model, num_steps_per_communication, window_prefix, pull_style):
         super(self.__class__, self).__init__(params)
@@ -1023,7 +1024,7 @@ def step(self, closure=None):
         return super(self.__class__, self).step(closure)
 
 
-class _DistributedPushSumOptimizer(torch.optim.Optimizer):
+class _DistributedPushSumOptimizer(Optimizer):
 
     def __init__(self, params, model, num_steps_per_communication):
         super(self.__class__, self).__init__(params)
diff --git a/bluefog/torch/topology_util.py b/bluefog/torch/topology_util.py
index b0fe38f2..0f5378d6 100644
--- a/bluefog/torch/topology_util.py
+++ b/bluefog/torch/topology_util.py
@@ -6,7 +6,7 @@
 import bluefog.torch as bf
 
 
-def _check_ranks(rank_list: List[Any], self_rank: int, size: int) -> [bool, str]:
+def _check_ranks(rank_list: List[Any], self_rank: int, size: int) -> Tuple[bool, str]:
     for rank in rank_list:
         if not isinstance(rank, int):
             return False, "contain element that is not integer."
@@ -21,7 +21,7 @@ def _check_ranks(rank_list: List[Any], self_rank: int, size: int) -> [bool, str]
 
 def InferSourceFromDestinationRanks(
     dst_ranks: List[int], construct_adjacency_matrix: bool = False,
-) -> Union[List[int], Tuple[List[int], np.array]]:
+) -> Union[List[int], Tuple[List[int], np.ndarray]]:
     """Infer the source ranks from destination ranks. This is collective communication call.
 
     Args:
@@ -49,7 +49,7 @@ def InferSourceFromDestinationRanks(
 
 def InferDestinationFromSourceRanks(
     src_ranks: List[int], construct_adjacency_matrix: bool = False,
-) -> Union[List[int], np.array]:
+) -> Union[List[int], np.ndarray]:
     """Infer the destination ranks from source ranks. This is collective communication call.
 
     Args:
diff --git a/examples/pytorch_benchmark.py b/examples/pytorch_benchmark.py
index c900797d..a68e2101 100644
--- a/examples/pytorch_benchmark.py
+++ b/examples/pytorch_benchmark.py
@@ -23,7 +23,7 @@
 import torch.nn.functional as F
 import torch.optim as optim
 import torch.utils.data.distributed
-from torchvision import models
+from torchvision import models # type: ignore
 import bluefog.torch as bf
 from bluefog.common import topology_util
 
@@ -66,7 +66,7 @@
 args.cuda = not args.no_cuda and torch.cuda.is_available()
 
 if args.dist_optimizer == 'horovod':
-    import horovod.torch as bf
+    import horovod.torch as bf # type: ignore
 
 bf.init()
 
diff --git a/examples/pytorch_mnist.py b/examples/pytorch_mnist.py
index 57ed59d8..163e3476 100644
--- a/examples/pytorch_mnist.py
+++ b/examples/pytorch_mnist.py
@@ -17,6 +17,7 @@
 from __future__ import print_function
 
 from bluefog.common import topology_util
+from typing import Tuple, List
 import bluefog.torch as bf
 import argparse
 import os
@@ -28,7 +29,7 @@
 import torch.nn.functional as F
 import torch.optim as optim
 import torch.utils.data.distributed
-from torchvision import datasets, transforms
+from torchvision import datasets, transforms # type: ignore
 
 sys.path.insert(0, os.path.abspath(
     os.path.join(os.path.dirname(__file__), "..")))
@@ -75,7 +76,7 @@
 
 if args.dist_optimizer == 'horovod':
     print("importing horovod")
-    import horovod.torch as bf
+    import horovod.torch as bf # type: ignore
 
 bf.init()
 
@@ -304,7 +305,7 @@ def test(record):
         )
     record.append((test_loss, 100.0 * test_accuracy))
 
-test_record = []
+test_record: List[Tuple[float, float]] = []
 for epoch in range(1, args.epochs + 1):
     train(epoch)
     test(test_record)
diff --git a/examples/pytorch_resnet.py b/examples/pytorch_resnet.py
index ae5c73d9..2b7fbf4d 100644
--- a/examples/pytorch_resnet.py
+++ b/examples/pytorch_resnet.py
@@ -30,9 +30,9 @@
 import torch.nn.functional as F
 import torch.optim as optim
 import torch.utils.data.distributed
-from torchvision import datasets, transforms, models
-import tensorboardX
-from tqdm import tqdm
+from torchvision import datasets, transforms, models # type: ignore
+import tensorboardX # type: ignore
+from tqdm import tqdm # type: ignore
 
 sys.path.insert(0, os.path.abspath(
     os.path.join(os.path.dirname(__file__), "..")))
@@ -96,7 +96,7 @@
 
 if args.dist_optimizer == 'horovod':
     print("importing horovod")
-    import horovod.torch as bf
+    import horovod.torch as bf # type: ignore
 
 # Bluefog: initialize library.
 bf.init()
diff --git a/scripts/.mypy.ini b/scripts/.mypy.ini
new file mode 100644
index 00000000..1fed97e1
--- /dev/null
+++ b/scripts/.mypy.ini
@@ -0,0 +1,5 @@
+[mypy]
+
+[mypy-matplotlib.*,networkx.*,pytest.*,numpy.*,tensorflow.*,psutil.*,torch.*]
+follow_imports = silent
+ignore_missing_imports = true
diff --git a/scripts/mypy.sh b/scripts/mypy.sh
new file mode 100755
index 00000000..0039ba79
--- /dev/null
+++ b/scripts/mypy.sh
@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+
+# Usage:  (under the root directory)
+#   tools/mypy.sh [--version]
+
+mypy_files=$(find bluefog examples test -name "*.py")
+mypy --config-file=./scripts/.mypy.ini "$@" ${mypy_files}
diff --git a/scripts/requirements_dev.txt b/scripts/requirements_dev.txt
new file mode 100644
index 00000000..5cf5d8d8
--- /dev/null
+++ b/scripts/requirements_dev.txt
@@ -0,0 +1 @@
+mypy==0.940
\ No newline at end of file
diff --git a/test/tensorflow_basics_test.py b/test/tensorflow_basics_test.py
index d867cfd0..8bbacd84 100644
--- a/test/tensorflow_basics_test.py
+++ b/test/tensorflow_basics_test.py
@@ -22,7 +22,7 @@
 import numpy as np
 import networkx as nx
 import pytest
-import tensorflow as tf
+import tensorflow as tf # type: ignore
 
 from common import mpi_env_rank_and_size
 import bluefog.tensorflow as bf
diff --git a/test/tensorflow_ops_test.py b/test/tensorflow_ops_test.py
index 19ae2341..e0a26756 100644
--- a/test/tensorflow_ops_test.py
+++ b/test/tensorflow_ops_test.py
@@ -20,9 +20,9 @@
 import itertools
 import numpy as np
 import os
-import tensorflow as tf
+import tensorflow as tf # type: ignore
 from bluefog.tensorflow.util import _executing_eagerly, _has_eager
-from tensorflow.python.framework import ops
+from tensorflow.python.framework import ops # type: ignore
 import warnings
 
 import bluefog.tensorflow as bf