What is the biggest upset in MMA history? IYO?

Probably not biggest upset when it come to victory, but in what happened, I'd have to say Fedor vs Arlvoski.

First people thought Fedor was getting schooled in boxing, then neither fighters were able to get the fight to the ground. Then Arlvoski suddenly threw a push kick out of blue and Fedor looked like he was about to fell down, but he bounced off the rope only to catch Arlvoski with a punches while Arlvoski was in mid air trying to go for a flying knee. That was just downright shocking.

Then after the fight, if you watch the fight in slow motion, you can see that Arlvoski didn't really landed much on Fedor at all which show how fast Fedor's reflex is.

But as for a actual match, I'd have to say first Fedor vs Nog. Before they fought, Nog was undefeated in 21 fight (he lose by split decision to Henderson, but many thought it should have went other way). Nog was basically Fedor during the time.

Also keep in mind, Nog's guard was the most feared one in MMA and no one wants to go to the ground with Nog. Fedor's game was mainly GnP and Sambo, so everybody thought Nog was going to submit Fedor.

But as soon as the fight hit to the ground, to everyone shock, Fedor dived right into Nog's guard and pounds on him while Nog was desperately trying to get Fedor out of his guard the whole fight. This have never been seen before and was totally unexpected.

So that's why I think this is probably one of the biggest upset, because of both, Nog was considered to be unbeatable at the time plus the way Fedor planned the game and put it to work was just totally unexpected and off.

For UFC Serra vs GSP 1 probably was the greatest upset of all time, but here's a few others that were close:

BJ Penn vs Hughes

Randy Couture vs Vitor Belfort 1

Tim Sylvia vs Ricco Rodriguez

Forrest Griffin vs Rampage (and Shogun for that matter)

Pete Williams vs Mark hunt

outside of UFC here's a couple that come to mind:

Cro-Cop vs Kevin Randleman

Gomi vs Nick Diaz

****************************

Nice Interesting take White Oni- but come on, how does Fedor vs Nog compare to Cro-Cop vs Randleman??

Hm... Mark Coleman vs. Maurice Smith? There was a time when some people had written of striking entirely. Coleman had been on a tear, destroying everyone in his way up until that point. Nobody was expecting an over-the-hill kickboxer to beat an Olympic wrestler at that point.