Various stakeholders within and outside of the organization are involved and participate in the outsourced IS development. As their interests and goals are different conflict arises in the process of their interaction, and it is an important factor affecting outsourcing success. The study focuses on the conflict between client users and vendor, i.e. main conflict in the outsourcing project, and examines the relationship of conflict with project success, the antecedents of conflict, and investigates the effectiveness of conflict resolution facilitation. An integrated conceptual model is developed grounded on is outsourcing, IS development, project management, and organizational behavior literature. Nine specific hypotheses on the conflict in outsourcing projects are proposed, and data collection and analysis are performed with 214 ISD outsourcing projects. Results indicated that the conflict between client users and vendor in outsourcing projects was found to have negative effects on project success. Main causes affecting conflict were examined, and relative influences of these causes were clarified. Recently internal IT personnel have been faced their role change with the increase of outsourcing, and the importance of their conflict resolution facilitation was highlighted.