Organizational Citizenship Behavior (OCB) is a valuable and useful behavioe that people arbitrarily and voluntarily express in their work place. (OCB) causes the staffs to prefer organizational and group interests over individual interests. In order to ensure organizational effectiveness, organizations need the cooperation, generosity, devotion and dedication, and sometimes, the staffs' additional effort. The discussion of gender differences in providing the level of organizational citizenship behavior have always been essential as a serious issue. The main purpose of the present research was the evaluation and identification of the staffs’ organizational citizenship behavior status and the investigation of gender effect on it in SAMEN Financial and Credit Institute in the Mazandaran Province. The method of the study was descriptive and correlative, and statistical population of present study included 148 staff of SAMEN Financial and Credit Institute in Mazandaran Province. The sample included 114 staff members. Data were collected by using Neihoff and Moormans’ (1993) standard questionnaire. We analyzed data by SPSS and LISREL software. Results showed that organizational citizenship behavior and all of evaluated dimensions, namely altruism, courtesy, sportsmanship, consciousness, and civic virtue placed in desirable status, and in exception of civic virtue, other dimension have a significant relationship with OCB. So, the female staff's OCB were much more desirable than male staffs impressively in all dimensions. Thus, the organizational could achieve higher level of productivity by paying more attention to and better planning on female staff's OCB.