The development of technology has enabled users to customize avatars using various combinations and appearances in MMORPG. This study investigated the loyalty and user immersion factors of MMORPG avatar customization, which included personal factors (self-esteem, self-efficacy) and environmental factors (challenging spirit, sense of unity). immersion factors were extracted and a research model was established by observing precedent studies conducted on Flow theory and on the development and transformation of avatars. This study established a theory based on a developed research model and by surveying the users of AION, a MMORPG game, who actively enjoyed avatar customization, after determining measurement items based on precedent studies. The research results showed that self-esteem, a challenging spirit and sense of unity must be enhanced to increase the customization immersion. In addition, research results showed that the degree of loyalty increased with higher customization immersion.