In online forums or virtual communities, the interactive relationships among members are similar to actual interpersonal social networks. Each member’s position in the social network affects the individual’s ability to resources for data or organized information. The position of members in networks also considerably affects the degree of opportunity to engage in activities that share individual knowledge. Therefore, beginning with the perspective of social network theory, this study uses social network analysis (SNA) to investigate the relationship between networks’ structures of forum and knowledge sharing as well as the interaction pattern among forum members. The results of this study show that network size closely relates to a member's centrality in the network. The larger the network is, the lower the member's centrality. A member with high centrality is likely to be either initiator of the discussion thread or expert in the forum’s subject. Observations of members’ in-centrality and out-centrality positions distinguish between these two potential roles.