Based on Bandura's work, the four sources of efficacy shaping were examined in regard to frequency and students' perception of importance in a computer-mediated, project-based high school classroom. In a context of group work where there was no designated leader, groups' collective efficacy was examined if it has any relationship with individual's leadership traits. In addition, the relationship between the existence of group-identified leader and the groups' collective efficacy as well as the relationship between the collective efficacy and the groups' performance outcome were examined. The results from survey and interview showed that mastery experience was the most frequent and powerful source of efficacy shaping among the four sources. Moreover, the groups with identified leader showed higher collective efficacy than the groups without leaders, and the groups' collective efficacy level showed positive correlation with the groups' performance outcome.