Nowadays, the internet is an unseperatable part of human life and instant messaging software becomes an important role of communication channel. There are 40 million global IM registration popularity who use IM and billions of messages are sent everyday. In Taiwan, there are 6.5 million registration popularity and IM is the third important internet appilication, besides WWW and e-mail. As IM has become more and more important, there was few academic researchs published. We investigated consumer IM adoption behavior by focus group interview and structural equation model analysis. After holding a focus group interview, some antecedent of adopting IM was found out. Then we integrated technology acceptance model with media richness, computer self-efficacy and subjective norm as the antecedents of perceived usefulness(PU) and perceived ease of use(PEU) and analysised this structural equation model with LISREL. We found that subjective norm is an antecedent of PU, media richness and computer self-efficacy are PEU¡¦s antecedents. Finally, we made some suggestion for practice and other researchers base on the result.