Nate Diaz says Georges St-Pierre was on steroids when he fought Nick Diaz back in 2013 -- and won't accept a fight with the UFC legend until he admits he was on the sauce.
Nate leveled the allegation in a fiery Facebook post ... saying, "Gsp cheated when he fought my brother he was on steroids."
FYI, GSP defeated Nick by unanimous decision while defending the UFC welterweight title at UFC 158.
Diaz continued ... "He was overweight and they let him be because we were in Montreal (GSP's home turf) and his boy gave nick a bad iv.