When it's going well, it can touch as high as 97 mph, and when mixed in with his nasty slider, he can be almost unhittable.
But Santana lost velocity on his fastball last season when he was battling elbow problems and saw his average fastball speed dip from 94.4 mph in 2008 to 92.3 mph in 2009.
It resulted in a down year for Santana, who posted a 5.03 ERA in 23 starts, which was a far cry from his dominant form in 2008, when he was named an All-Star and finished the season with 16 wins, a 3.49 ERA and 214 strikeouts in 219 innings.
Santana is now trying to get back to that form, and despite the fact that he allowed five runs on seven hits over 3 2/3 innings against the Giants' Minor League squad on Thursday, he feels like's close to that form from two years ago.
"I think it's better, like it was before," Santana told reporters on Thursday. "Way better, because I can feel the difference throwing the ball. Even my changeup I can throw for a strike or a ball and my slider, it's the same thing. And I'm more mature."
His performance on Thursday was a bit surprising considering he had previously thrown five scoreless innings against Major League clubs with five strikeouts and no walks in those two outings.
But on the bright side, he did strike out five more batters on Thursday with no walks and came out feeling fine.
"Everything felt good," Santana said. "Just not good results.
He allowed a three-run homer to former White Sox top prospect Joe Borchard on a back-door slider and two RBI doubles in the third inning.
But Santana isn't worrying about the results until he stretches his outings to at least five innings so that he can build up his stamina and set up hitters like it's a regular-season game.
"My next outing is going to be five innings and you have to get ready for the season," Santana said. "So I'll be pitching and trying to locate everything like I would during the season."
Rhett Bollinger is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.