Outsourcing of information system (IS) functions is growing rapidly. Many researches relevant to the IS functions outsourcing, from the economic viewpoint, used the resource-dependence theory, transaction-cost theory, and agent cost theory. Explaining the relationship between organizations from a purely economic point of view is unjustifiable because inter-organizational relationships form from the social learning experiences based on specific sequential interactions. This study focuses on the working relationship between the outsourcing service receiver and the service provider in the outsourcing on-going stage based on a social perspective. The purpose of this study intends to use the social theories to investigate the influencing factors of IS outsourcing partnerships between the firm and the service providers. In order to validate the research model, this study integrated case study and survey research methods, or what has been called 'triangulation' to verify the research model. The results of the study provide a research model of the IS outsourcing partnerships to the follower researchers, and provide the suggestions to the firm to ensure the success of the IS functions outsourcing.