Background: The organizational citizenship behavior is generally known as the important factor relevant to the organizational effectiveness. This research examined the mediating effect of the organizational citizenship behavior of hospital employees on the organizational effectiveness. Methods: Data were collected from 1,112 employees located in city of Seoul, Kyunggi and Chungnam province through self-administered questionnaires. Collected data were analyzed using IBM SPSS ver. 20.0, frequency analysis, t-test, analysis of variance, regression analysis, and path analysis. The main findings of the study are as follows. Results: First, it was found that many characteristics variables of personality, job, and relationship together affected organizational citizenship behavior of hospital employees. Especially, the following variables of negative affectivity, desire for growth, job value, job significance, and job security were found to have significant effect on the organizational citizenship behavior of hospital employees. Second, the results of path analysis showed that, through the mediating effect of organizational citizenship behavior, personality variables of positive and negative affectivity, and desire for growth, job characteristics variables of job value, job significance, and job security, and relationship variables of organizational support and task interdependence, had significant total effects on the level of job satisfaction of hospital employees. Conclusion: As a result, the organizational citizenship behavior seems to have both direct and indirect effects on the organizational effectiveness of hospital employees. Based on above findings, some theoretical and practical implications were discussed.