In virtual worlds, users create their own disembodied characters called avatars. They project themselves through avatars, altering their physical appearances. Despite the recent increasing interests in avatar and self relationship and its psychological aftermath, not much research fully illustrates about the motivations and the consequences. This paper mainly discuss about avatar-real self and self-efficacy of MMORPG players. A total number of 675 South Korean game players participated in an online survey. We explored the relationship between avatar-self similarity and game players' performance. Also, we explored the relationship between mastery experience and players' general self efficacy. The results showed that avatar-self similarity affected the mastery experience and mastery experience influenced players' general self-efficacy. In conclusion, we added a layer of explanation of reasons of avatar-self similarity and its attitude or behavior changes. In terms of self-efficacy, more serious implications of playing online games are also suggested.