Social commerce greatly changes buyers’ purchase behavior nowadays. Understanding the psychological mechanism of consumers’ purchase behavior is important for both buyers and sellers in social commerce platform. This study will examine the nature and role of psychological contract (PC) that is an utmost important variable in social commerce context. The psychological contract comprises a consumer’s beliefs about the reciprocal obligations that exist between him/her and sellers or other social members. Specifically, we propose that consumers may make purchase decisions by two processes, in which two types of PC--transactional contract and relational contract are the key mediators. In addition, we further discuss the antecedents (platform website ethics, the reliability of social elements) and consequences (trust and satisfaction in the platform) of PC, and their impacts on purchase intention. We plan to empirically test our model by collecting survey data from the users from the largest social commerce website in China—Taobao.