A previous study showed that the effects of the Big Five Personality Domains on Organizational Citizenship Behavior (OCB), which is informal voluntary behavior by employees, are indirect and that there is a need for clarification of the mediating mechanism between Big Five Personality Domains and OCB. This study focused on “political skill” which is the ability to exercise organizational politics, and examined the mediating effect of political skill in the effects of the Big Five Personality Domains on OCB. Participants were 309 full-time workers in a Japanese enterprise (231 men and 78 women; mean age 44.20 years, SD=8.16). Our results showed that “apparent sincerity” and “interpersonal influence”, subscales of political skill are the intervening variable which explaining the partly process of the Big Five Personality on OCB.