This research is to investigate the impact of informal communication of smart-phone user while at work on performance. To do this, we proposed the research model including informal offline communication represented by tea time and informal online communication typified by Kakao talk derived from water-cooler effect. The results of this empirical study is summarized as followings. First, Kakao talk with insiders has a positive effect on performance only in male group, and Kakao talk with outsiders has a negative effect on performance in both male and female group. Secondly, tea time has a positive effect on performance in group with high task interdependence, whereas Kakao talk with insiders has a positive effect on performance in group with low task interdependence. The findings have significant implications for policy development to regulate the wise smart-phone use in organization.