“Organizational Citizenship Behavior (OCB)” was first created in 1983 and has been one of important issues in business organizations. However, most studies served OCB as dependent variable and discussed the predictors of OCB. Followed by Bolino, Turnley, and Bloodgood (2002), this study viewed OCB as instrumental variable and discussed its effects. We found that both OCB and Social Capital can predict self-efficacy and happiness. However, OCB’s effects through Social Capital are limited. We substantiated that the effects of Social Capital on self-efficacy and happiness are mediated by OCB.