This study explores continual usage of the knowledge management systems (KMS) by employees in organizations. Based on IS continuance research that emphasizes usefulness and satisfaction, we propose that to understand KMS usage, scholars need to examine not only technical but also social and organizational aspects of KMS in determining the user's attitude. Through an intensive case study in an international IT company, we formulate a theoretical model to explain KMS usage in organizations. The model employs social capital theory to examine how social relationships (trust, tie strength, and shared norms) may affect the user's attitude toward KMS usage, and organizational behavior theory to identify the influence of organizational facilitating conditions such as rewards, job requirements, and other organizational assistances on attitude formation. A large-scale survey will further be undertaken in organizations that have implemented KMS for more than half a year. The survey results are expected to conform to the proposed model. Insights for research and practice as well as future directions are discussed.