diff --git a/icartt/dataset.py b/icartt/dataset.py
index 3fddd3fcae201a9e3952ce879f366629bf336529..c620eb2ae4a31e32d3c543d9474b371a1efc5381 100644
--- a/icartt/dataset.py
+++ b/icartt/dataset.py
@@ -396,7 +396,7 @@ class Dataset:
 
         # read data if f is not None
         if f is not None:
-            if isinstance(f, (str, unicode)):
+            if isinstance(f, str):
                 self.input_fhandle = open(f, 'r')
             else:
                 self.input_fhandle = f