The growing use of information technology and the Internet has led to the emergence of professional virtual communities (PVCs). Why are PVC members willing to spend time and energy to provide valuable knowledge to others? This study extended Icek Ajzen’s(1991) theory of planned behavior (TPB) to analyze what determines PVC members’ willingness to share knowledge. Empirical data were collected from three IT-related PVCs in Taiwan, and the structural equation modeling (SEM) was performed to verify the fit of the proposed model. Based on 423 usable samples were obtained, the results showed that knowledge sharing intentions would be significantly affected by, in order of importance, attitude toward knowledge sharing, perceived behavioral control of knowledge sharing, and subjective norm of knowledge sharing. The factors affecting attitude toward knowledge sharing were, in order of importance, perceived ease of use, perceived usefulness, enjoyment in helping others, knowledge self-efficacy, compatibility, and trust. The major factor that affected the subject norm of knowledge sharing was peer influence. Factors that affected perceived behavioral control of knowledge sharing were, in order of importance, knowledge self-efficacy, resource availability, and perceived ease of use.