At the main menu, select the "Unlock Code" option, then enter one of the following case-sensitive codes to activate the corresponding cheat function:

Play as Brock Lesnar

Enter "BROCKCHAMP" as a code to unlock Brock Lesnar.

Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Pound-for-Pound Champion (100 points): Win the UFC Championship in each weight class in Career Mode.
Undisputed Champion (25 points): Win the UFC Championship with a fighter in Career Mode.
Hall of Famer (25 points): Enter the Hall of Fame with a fighter during Career Mode.
Call me "No Touch!" (75 points): Complete Career Mode and retire undefeated.
Where’s my bonus check? (20 points): Obtain the KO, Submission and Fight of the Night Awards with a fighter in Career Mode.
That was easy! (20 points): Win in under 20 seconds of the 1st round against a COM UFC Fighter in Exhibition or Career Mode.
Student of the game. (25 points): Complete all tasks from the Tutorial Mode.
Wax Off! (20 points): Escape 20 submissions by technical escape against a COM UFC Fighter in Exhibition or Career Mode.
Trophy Collector (25 points): Obtain all the trophies in Career Mode with a fighter.
UFC Historian (20 points): Unlock all movies from the Classic Fights Mode.
World Class Striker (30 points): Win by KO without using any grappling against a COM UFC Fighter in Exhibition or Career Mode.
World Class Grappler (30 points): Win by Submission using no striking against a COM UFC Fighter in Exhibition or Career Mode.
Roster Run-through (40 points): Complete one fight in Exhibition Mode with each UFC fighter.
White Belt (10 points): Win a Ranked Match with a CAF or UFC fighter in Xbox LIVE.
Blue Belt (20 points): Win 15 Ranked Matches with a CAF or UFC Fighter in Xbox LIVE.
Purple Belt (30 points): Win 30 Ranked Matches with a CAF or UFC Fighter in Xbox LIVE.
Brown Belt (50 points): Win 50 Ranked Matches with a CAF or UFC fighter in Xbox LIVE.
Black Belt (100 points): Win 100 Ranked Matches with a CAF or UFC fighter in Xbox LIVE.
Wax On! (30 points): Win 15 Ranked Matches with a CAF or UFC fighter by submission in Xbox LIVE.
Papa Said Knock You Out! (30 points): Win 15 Ranked Matches with a CAF or UFC fighter by KO in Xbox LIVE.
Gold Digger! (30 points): Obtain all the gold medals in a Xbox LIVE Ranked Match Weekly Mission.
You really like me! (20 points): Gain a million or more fans in a Xbox LIVE Ranked Match.
Ooh, in ya face! (25 points): Win a fight by flash KO in a Xbox LIVE Ranked Match with a CAF or UFC fighter.
Original fighter (25 points): Create A Fighter and use the CAF in Exhibition or Xbox LIVE.
Red Belt (100 points): Reach level 50 or higher in a Xbox LIVE Ranked Match.

Additionally, there are five secret achievements:

Two of my favorites! (15 points): Watch the Octagon Girl cutscene 15 times without skipping it.
Wanted Flying Mouthpieces (15 points): Mouth piece travels more than 3 meters when knocked out.
Ultimate Highlights (15 points): Dominate the opponent in all 3 scenes of a replay segment.
Ultimate Knockout (15 points): From the gassed state, KO an opponent in any mode.
Ultimate Submission (15 points): From the gassed state, Submit an opponent in any mode.