diff --git a/deepdoc/vision/operators.py b/deepdoc/vision/operators.py
index b72ba6d2..305a028b 100644
--- a/deepdoc/vision/operators.py
+++ b/deepdoc/vision/operators.py
@@ -102,7 +102,7 @@ class StandardizeImage(object):
 
 
 class NormalizeImage(object):
-    """ normalize image such as substract mean, divide std
+    """ normalize image such as subtract mean, divide std
     """
 
     def __init__(self, scale=None, mean=None, std=None, order='chw', **kwargs):