The purpose of this study is to investigate how group attachment security impacts organizational citizenship behavior through individual sense of power. To strengthen causal relationships and rule out alternative explanations, we conducted an experiment based on university students. We collected data from 140 students in a university. Participants were randomly assigned to either the secure group attachment or control conditions. In our study, we find that (1) the securely group attachment was positively related to individual sense of power than control conditions, which in turn positively influenced organizational citizenship behavior. (2) In addition, individual sense of power fully mediated the relationship between group attachment security and organizational citizenship behavior. This study expands literature about group attachment, sense of power, and OCB. The key contribution lies in introducing sense of power as a primary mediator of the effect of securely group attachment on OCB. Also, our research indicates that many organizations could train managers to develop levels of flexible culture and to support communications. We discuss further academic and practical implications for understanding how secure group attachment affects OCB through individual sense of power.