Knowledge Management System is defined as an Information system to support the integrated knowledge management process for maximizing value of the knowledge assets in organizations. From that point of view, the system performance - the usage level of the system and user satisfaction - can be an available measure of the knowledge management system success. Many previous research were focused on the application of a new technological component for knowledge management processes or the construction of the overall system architecture in spite of the importance of the system performance. The purpose of this study is to Identify the factors affecting the knowledge management system performance. To do this, we conducted an exploratory case studies of eight companies. We applied theories of information system success to knowledge management system and investigated some specific strategic components as additional affecting factors. Finally, we proposed research model and ex propositions regarding the factors affecting the knowledge management system performance. The implication of the findings and future research directions were discussed.