In recent years, extensive studies of many interesting aspects of virtual community dynamics promoted a better understanding of this area. One of the most challenging problems facing builders of virtual communities is the design of incentive mechanisms that can ensure user participation. However, running virtual community experiments in the real world is expensive, and requires a great deal of motivation from users. In this paper we advocate a system dynamics approach to simulate the overall behaviors of participants in the communities, which can provide insights into the user motivation process, incentive mechanism evaluation and community development. A simulation model for a virtual community called Comtella is presented, and the results are very promising.