Virtual communities (hereinafter as VCs) emerged as a new form of business model in electronic business field. Understanding motivations of members' participation is essential to virtual community organizers. Arguing that interpersonal relationship needs are important to drive members' participation, this paper used FIRO (Fundamental Interpersonal Relationship Orientation) as framework to explore why people participate in virtual communities. Data was collected in three large representative VCs and analyzed with ANOVA. It was found that people obtain information in virtual communities because they want to fulfill two kinds of needs-need for inclusion and need for affection; and people give information in virtual communities because they want to fulfill three types of needs-need for inclusion, need for control, and need for affection. This study thus contributed to the current knowledge of virtual communities by providing the integrative theoretical explanation and valid empirical results. This result is also meaningful to VC organizers.