diff --git a/scrapyd/website.py b/scrapyd/website.py
index 9442fc8a..cfcc557a 100644
--- a/scrapyd/website.py
+++ b/scrapyd/website.py
@@ -132,7 +132,7 @@ def __init__(self, root, local_items):
         self.prefix_header = root.prefix_header
 
     cancel_button = """
-    <form method="post" action="{base_path}/cancel.json">
+    <form method="post" onsubmit="return confirm('Are you sure?');" action="{base_path}/cancel.json">
     <input type="hidden" name="project" value="{project}"/>
     <input type="hidden" name="job" value="{jobid}"/>
     <input type="submit" style="float: left;" value="Cancel"/>