A previous study showed that the effects of the Big Five Personality Domains on OrganizationalCitizenship Behavior (OCB), which is informal voluntary behavior by employees, are indirect and thatthere is a need for clarification of the mediating mechanism between Big Five Personality Domains andOCB. This study focused on "political skill" which is the ability to exercise organizational politics, andexamined the mediating effect of political skill in the effects of the Big Five Personality Domains onOCB. Participants were 309 full-time workers in a Japanese enterprise (231 men and 78 women; meanage 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 BigFive Personality on OCB.