diff --git a/tox.ini b/tox.ini
index c62cef60..0ac4ca73 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,4 +1,7 @@
 [tox]
+# downgrade virtualenv to 20.21.0 have python 3.6 support
+# https://github.com/tox-dev/tox/issues/2989
+requires = virtualenv<20.22.0
 envlist =
     py36
     py37