How to attain competitive advantage in network is a new focus in strategic management. Knowledge is an important source for firm's continual competitive advantage. By analyzing the mechanism that how the three dimensions of social capital, i.e. structure, cognition and relation, influence the capacity and transfer of knowledge in network, we formed a model for firm to attain competitive advantage in network. Followed by a case study, we pointed out that firms must build up their management capability of knowledge and network to match with the evolving social capital.