diff --git a/README.md b/README.md
index dca933a..f4bdef4 100644
--- a/README.md
+++ b/README.md
@@ -79,7 +79,7 @@ An image decode operator takes an image path as input. It decodes the image back
 
 
 
-**Returns**: *towhee.types.Image*
+**Returns**: *towhee.types.Image (a sub-class of numpy.ndarray)* 
 
 ​	The decoded image data as numpy.ndarray.