In this study, we integrate the theories of self-efficacy, outcome expectancy, and social capital to identify individual and contextual factors that may exert effect on professional teachers' knowledge sharing behaviors in an Internet-based knowledge community. Data collected from 441 members of this community reveal that knowledge sharing self-efficacy has significant influence on knowledge sharing outcome expectancy, anxiety, and knowledge sharing behaviors. In addition, relational identity, i.e., the degree of social capital among members, is found to have positive effects on knowledge sharing self-efficacy and knowledge sharing behaviors. The implication of this study is that both the individual perceptions toward knowledge sharing and the interpersonal relational development are important predictors of ongoing knowledge sharing activities. The organizers of Internet-based knowledge communities should therefore focus on the development of members' resilient self-efficacy, favorable outcome expectancy, and strong relational identity if they wish knowledge sharing to be effective.