Organizational citizenship behavior (OCB) is one of the most important issues that recently have gained special attention in studying organizations including hospitals. Recent studies show that employees demonstrate special interest in OCB and it can be influenced by their demographic characteristics. This study was designed to investigate the relationship between OCB and demographic characteristics (including sex, marital status, academic qualification, type of profession, age, salary and wages, ethnicity, type of employment, department/office, job position, and duration of employment,) in the selected Tehran's hospitals. The study was descriptive cross-sectional. The sample includes 333 employees of the selected hospitals chosen based on multistage sampling. Hypothesis was tested through Mann-Whitney and Kruskal Wallis tests and one-way ANOVA. The results showed that, all demographic characteristics, except ethnicity, had significant effect on OCB. The relationship between demographic characteristics and OCB was analyzed and specific managerial strategies and implications were suggested.