PURPOSE The purpose of this study was to identify the predictors of Organizational Citizenship Behavior (OCB) among hospital nurses. METHODS Participants in this study were 231 nurses from two hospitals. The self-reported questionnaire was used to assess the level of self-efficacy, social support and OCB. The data were analyzed using descriptive statistics, Pearson's correlation coefficient, t-test, ANOVA and multiple regression. RESULTS The mean score of OCB was 4.82, self-efficacy was 4.65, and social support was 4.56. The OCB was statistically significant according to position (t=-1.97, p=.049). The OCB was positively correlated with self-efficacy (r=.558, p<.001) and social support (r=.245, p<.001). The self-efficacy and social support explained 33.0% of the variance for OCB. CONCLUSION The findings suggest that developing programs to improve self-efficacy and social support might be useful. Furthermore, more studies are needed to explore variables that influence nurses' Organizational Citizenship Behavior.