In recent years, peer-to-peer systems have become more and more popular, especially with some successful applications like Napter, Kazaa and edoney. However, how to motivate user participation in peer-to-peer systems remains an open question for researchers. If few users are willing to participate in the community or make contributions to it, the peer-to-peer system will never become a successful one. To address the problem, this paper proposes a motivation strategy based on the theories of social psychology. The main idea is to introduce a set of hierarchical memberships into p2p communities and reward active users with better quality of services. We have applied this strategy to a p2p system called Comtella and launched a study to test its effectiveness. The preliminary results of the study have show that the motivation strategy is capable of stimulating the users to participate more actively and make more contributions to community. Keywords Motivation, peer-to-peer system, social ...