The one under his eye, or the huge x on his chest?
See Also
Luffy and Hanco*ck's child episode 2: Luffy visits Amazon LilyOne Piece: Oda Teases The Mystery Behind Zoro's Left EyeLuffy's scarIn which episode does Luffy get his scar?The one under his eye was gotten when he was trying to prove to Shanks he was fearless and used a knife to do so himself. Can't remember the episode but it was in a flashback.
The huge X on his chest was given to him by Akainu during the MF war when Jinbei tried to escape with him.
