diff --git a/authlib/oauth2/rfc6749/requests.py b/authlib/oauth2/rfc6749/requests.py
index 7f6a7091..625e37d4 100644
--- a/authlib/oauth2/rfc6749/requests.py
+++ b/authlib/oauth2/rfc6749/requests.py
@@ -76,7 +76,7 @@ def response_type(self) -> str:
 
     @property
     def grant_type(self) -> str:
-        return self.form.get('grant_type')
+        return self.data.get('grant_type')
 
     @property
     def redirect_uri(self):