In Chapter 3, George, Lennie, and Candy are sitting in the bunkhouse, when Slim and Curly enter. Curly (the boss' son) has accused Slim of trying to seduce Curly's wife, and was obviously wrong in his accusation. The men in the bunkhouse begin teasing Curly, mocking him. Curly turns to see Lennie smiling (Lennie is not even paying attention to the other men; he is still thinking about the dream house with the rabbits). Curly thinks Lennie is laughing at him, so he attacks Lennie. Curly punches Lennie in the nose, then knocks the wind out of him with a blow to the stomach. Finally, George tells Lennie to fight back, and when Lennie does, he grabs Curly's fist and crushes it. When Lennie finally lets go, he is very upset because he is a peaceful person, and he doesn't like to harm people; his only concern is the rabbits and the dream home.
I was unsure exactly which part of the book you were referring to, so if this does not answer your question, post another with more specific information.