The vendor firm’s body of knowledge (BoK) has been considered to be vital in IT outsourcing success since outsourcing is considered as flow of knowledge especially from vendor firms to client firms. However, previous researches mostly focused on the client firms’ perspective and less attention on vendor firms and their knowledge bases. In this paper, we investigated the impact of vendor IT firm’s body of knowledge on the success of offshore IT outsourcing through an interaction process based on knowledge based view (KBV) and interaction process model (IPM). At first, vendor firm’s various knowledge bases (that comprise firms’ capability) are identified based on the knowledge based view and then, an interaction process of outsourcing relationship is created as a first order impact of various knowledge bases. It is assumed that vendors share their knowledge by making an outsourcing relationship with clients through an interaction process, which ultimately impact on the success of IT outsourcing.