With the Internet changing from era Web1.0 of computers’ interaction to era Web2.0 of users’ interaction, the development of transactional community has become the key solution to combine economic value with social value in social commerce. Transactional community is different from traditional social community in its relationship establishment because of the unique motivations of their members. Based on network data from Taobao.com, this research analyzed the network closure mechanism in transactional community and the results showed: (1) Because of high cost of social interaction and the risk of inefficiency of relationship, members in transactional community would choose to avoid reciprocity; (2) The evolution of transactional community is mainly driven by informational social influence rather than normative social influence, as a consequence, number of contagion paths has a negative effects on getting relationship embeddedness from the other members; (3) The relationship establishment in transactional community mainly comes from mutual acquaintances or mutual activities.