Diaz is a Brazilian jiu-jitsu black belt under Cesar Gracie whom he has been training with since he was a teenager. He is accomplished in both gi and no-gi jiu-jitsu. Some of the major titles he has won include the US Purple Belt Open in 2004 and a Pan-American Brown Belt Medium Weight Division title in 2005. He also owns a victory over renowned grappler Jorge Patino in competition via kneebar. He is known for having a modified BJJ style that suits his MMA career very well. Diaz lists his favorite submission as the kimura.