Past research has shown that most offshore IS outsourcing projects were failed. Communication and coordination problems caused by culture differences between clients and vendors are important reasons for the failures. The theory of boundary spanning capability provides a new theoretical perspective for research of coordination and communication mechanisms between clients and vendors in offshore IS outsourcing. This paper takes culture differences as mediator to study the impact mechanisms of vendors’ boundary spanning capability on success of offshore IS outsourcing. The research model is empirically tested on a sample of 102 offshore IS outsourcing projects from 20 vendors in China. The results suggest that vendors’ boundary spanning capability significantly improves outsourcing success. Moreover, vendors’ boundary spanning capability has a negative impact on culture differences and culture differences negatively influence the outsourcing success. Finally, we find that culture differences mediate partly the relationship between boundary spanning capability and outsourcing success.