Abstract： The influence factors of Information Systems (IS) bidirectional knowledge transfer which between IS outside supplier and IS demander in the development mode of IS application outsourcing or pruchasing application software package are explored. Firstly, drawing from the knowledge transfer and IS literatures, a theoretical model is developed, the model presumes that IS bidirectional knowledge transfer is influenced by knowledge-related (shared understanding), communication-related (bidirectional trust, communication capabilities of both sides) factors.Then, a non-intervention empirical researchis adopted to simulate real situation, and data is collected based on an internet forum discussing IS knowledge for regression analysis. The analysis provides support for 10 of the 12 hypotheses and confirms two mediating relationships.