Knowledge has been recognized as one of the most important valuable resources for organizational competitiveness in our information society. Consequently, many organizations are implementing knowledge management systems. Yet, as the old saying ¡§actions speak louder than words¡¨ suggest, they are finding out that the actual level of knowledge sharing has lacked behind the espoused excitement. In this study, we attempt to investigate the psychological mechanisms that underlie people¡¦s inaction in knowledge sharing knowledge even though they understand the benefit. Specifically, we focus on the relationship between individual¡¦s cognition and behavior of knowledge sharing in a virtual community. The site that we choose for observation is SCTNet (http://sctnet.edu.tw/index.htm), which is a website designed for sharing knowledge by teachers of primary and middle-high schools. It allows them to upload/download files and to provide electronic forums for sharing their innovations. This study applies the Theory of Planned Behavior and the Action Control Theory to examine factors that may influence individual¡¦s intention and behavior of knowledge sharing in a virtual community. The results are useful to understand the dynamics of knowledge sharing in virtual community and provide insights for mangers of knowledge management systems. Keywords¡GKnowledge Management, Theory of Planned Behavior, Action Control Theory