Based on an in-depth case study, this article studies a collaborative IT project (one IT service vendor and three clients) involving intensive knowledge integration activity. It explains how social capital (e.g. relational, cognitive, structural), absorptive capacity (e.g. exploratory, exploitative, transformative) interact with each other with the support of organizational culture (e.g. collective, cooperative, compatible) to ensure the success of cross-organization knowledge integration information system project. Dividing the project process into three phases, we discuss in detail that in initiation phase, relational social capital initiates the exploratory absorptive capacity while exploratory absorptive capacity enhances the relational social capital in return; in implementation phase, cognitive social capital and exploitative absorptive capacity are utilized and synthesized to keep the smoothness of the implementation process; and in assimilation phase, transformative absorptive capacity leads to the reconfiguration of structural social capital while the reconfigured structural social capital in turn reinforces the transformative absorptive capacity. Within these three phases, we identified the different roles that culture plays as norm, facilitator and harmonizer. Key Words: knowledge integration, social capital, absorptive capacity, organizational culture.