Integrating social presence theory and social identity theory, this study brings system design and social influence aspects together to explain their joint effects on knowledge contribution in virtual communities (VCs). Different from most prior information systems (IS) research that adopts a uni-dimensional approach and restricts social presence to be the subjective nature of media, we developed and empirically tested a model explaining the effects of multi-dimensional social presence on social identification processes and knowledge contribution. An online survey was conducted with four different VCs of interest. The results showed the difference in relative contribution of social presence dimensions on social identity as well as knowledge contribution. Both practical and theoretical implications are discussed. (Contains 5 tables, 2 figures and 2 notes.)