Supply chain integration has been considered to be a source of competitive advantage for firms as it improves relationships and the flow of information and resources both between internal functions in an organization and between supply chain partners. this study examines the relationship between internal integration and functional performance. It also examines the three key antecedents to internal integration: communication, leader support and conflict. A questionnaire survey is used to collect data from a sample of 152 production managers. The data are analyzed using regression. The results indicate that communication, conflict and leader support are antecedents of internal integration and also impact functional performance. Internal integration is also found to mediate the relationships between communication and leader support with functional performance, but not for the relationships between conflict and functional performance. This is also the contribution of the paper, which is the first to examine and suggest these mediating effects between communication, leader support, internal integration and functional performance.