This study proposes a conceptual model based on Commitment-Trust theory with extended variables to examine the impact of commitment and trust on member behaviors in virtual communities. The members of Programmer Club, a representative professional community in Taiwan, were chosen to participate in the survey, and 386 usable responses were collected in three months. Two groups were divided based on usage, namely, heavy user and light user. Structural Equation Model (SEM) were used to test the model, the findings show that relationship commitment is the strongest predictor of member various behaviors (acquiescence, propensity to leave, discussion, and subscription). Implications are proposed in the final section.