This research examines the causal relationships between supervisor trust and organizational citizenship behavior(OCB) and the mediating role of organizational trust on OCB. Using the sample of 282 small and medium sized enterprises’ employees, this study analyzes three alternative hypotheses centered around supervisor trust, organizational trust, and OCB. The results show that supervisor trust has a strong positive effect on OCB. Second, organizational trust is positively related to OCB. Third, this research proves that partial mediating effects of organizational trust exist between supervisor trust(relational trust) and OCB(civic virtue, sportsmanship). Finally, this research proves that a full mediating effect of organizational trust exists between supervisor trust(calculative trust) and OCB(sportsmanship). This study recommends supervisor trust, organizational trust be valued in order to increase OCB vital for organizational effectiveness or productivity in small-medium sized manufacturing enterprises.