diff --git a/mmcv/runner/base_runner.py b/mmcv/runner/base_runner.py
index f78f1582f3..e28179d404 100644
--- a/mmcv/runner/base_runner.py
+++ b/mmcv/runner/base_runner.py
@@ -330,6 +330,20 @@ def resume(self,
 
         self._epoch = checkpoint['meta']['epoch']
         self._iter = checkpoint['meta']['iter']
+
+        # Re-calculate the number of iterations when resuming
+        # models with different number of GPUs
+        if 'config' in checkpoint['meta']:
+            config = mmcv.Config.fromstring(
+                checkpoint['meta']['config'], file_format='.py')
+            previous_gpu_ids = config.get('gpu_ids', None)
+            if previous_gpu_ids and len(previous_gpu_ids) > 0 and len(
+                    previous_gpu_ids) != self.world_size:
+                self._iter = int(self._iter * len(previous_gpu_ids) /
+                                 self.world_size)
+                self.logger.info('the iteration number is changed due to '
+                                 'change of GPU number')
+
         if 'optimizer' in checkpoint and resume_optimizer:
             if isinstance(self.optimizer, Optimizer):
                 self.optimizer.load_state_dict(checkpoint['optimizer'])
diff --git a/mmcv/utils/config.py b/mmcv/utils/config.py
index 17e44e7cff..ab4eb02aa6 100644
--- a/mmcv/utils/config.py
+++ b/mmcv/utils/config.py
@@ -5,6 +5,7 @@
 import shutil
 import sys
 import tempfile
+import warnings
 from argparse import Action, ArgumentParser
 from collections import abc
 from importlib import import_module
@@ -253,6 +254,31 @@ def fromfile(filename,
             import_modules_from_strings(**cfg_dict['custom_imports'])
         return Config(cfg_dict, cfg_text=cfg_text, filename=filename)
 
+    @staticmethod
+    def fromstring(cfg_str, file_format):
+        """Generate config from config str.
+
+        Args:
+            cfg_str (str): Config str.
+            file_format (str): Config file format corresponding to the
+               config str. Only py/yml/yaml/json type are supported now!
+
+        Returns:
+            obj:`Config`: Config obj.
+        """
+        if file_format not in ['.py', '.json', '.yaml', '.yml']:
+            raise IOError('Only py/yml/yaml/json type are supported now!')
+        if file_format != '.py' and 'dict(' in cfg_str:
+            # check if users specify a wrong suffix for python
+            warnings.warn(
+                'Please check "file_format", the file format may be .py')
+
+        with tempfile.NamedTemporaryFile('w', suffix=file_format) as temp_file:
+            temp_file.write(cfg_str)
+            temp_file.flush()
+            cfg = Config.fromfile(temp_file.name)
+        return cfg
+
     @staticmethod
     def auto_argparser(description=None):
         """Generate argparser from config file automatically (experimental)"""
diff --git a/tests/test_utils/test_config.py b/tests/test_utils/test_config.py
index 013bb2f5c5..5abafe80b8 100644
--- a/tests/test_utils/test_config.py
+++ b/tests/test_utils/test_config.py
@@ -161,6 +161,31 @@ def test_fromfile():
         Config.fromfile(osp.join(data_path, 'color.jpg'))
 
 
+def test_fromstring():
+    for filename in ['a.py', 'a.b.py', 'b.json', 'c.yaml']:
+        cfg_file = osp.join(data_path, 'config', filename)
+        file_format = osp.splitext(filename)[-1]
+        in_cfg = Config.fromfile(cfg_file)
+
+        out_cfg = Config.fromstring(in_cfg.pretty_text, '.py')
+        assert in_cfg._cfg_dict == out_cfg._cfg_dict
+
+        cfg_str = open(cfg_file, 'r').read()
+        out_cfg = Config.fromstring(cfg_str, file_format)
+        assert in_cfg._cfg_dict == out_cfg._cfg_dict
+
+    # test pretty_text only supports py file format
+    cfg_file = osp.join(data_path, 'config', 'b.json')
+    in_cfg = Config.fromfile(cfg_file)
+    with pytest.raises(Exception):
+        Config.fromstring(in_cfg.pretty_text, '.json')
+
+    # test file format error
+    cfg_str = open(cfg_file, 'r').read()
+    with pytest.raises(Exception):
+        Config.fromstring(cfg_str, '.py')
+
+
 def test_merge_from_base():
     cfg_file = osp.join(data_path, 'config/d.py')
     cfg = Config.fromfile(cfg_file)