Several researchers suggest that tacit knowledge sharing among employees is a process of social interaction by nature. Accordingly, the perspectives of social capital and behavioral control are employed in this study to investigate an individual’s tacit knowledge sharing and behavior within a workgroup. This study collects data through a multi-informant questionnaire design. There are three areas of interesting results. First, results show that tacit knowledge sharing intention and behavior can be induced by affect-based trust through organizational citizenship behavior. Second, internal control has positive effect on tacit knowledge sharing intention, but the relationship between internal control and tacit knowledge sharing behavior cannot gain support. Third, external control positively moderates the relationship between tacit knowledge sharing intention and behavior. It is interesting to note that tacit knowledge sharing intention does not necessarily lead to tacit knowledge sharing behavior unless the moderating effect of external control is taken into account. These findings and their implications are also addressed.