Political self-efficacy has been constructed based on social cognitive theory to describe the beliefs that employees hold on their own political traits with which they can achieve personal goals in political environment successfully. Employees’ perceptive comparison of their own political traits and the political environment that indicates the general level of organizational politics can be regarded as the major source of political self-efficacy. The more the confidence that employees have in their competency, the higher their political self-efficacy will be. Organizational citizenship behavior (OCB), one of the facilitators of organizational effectiveness, describes the out-role behaviors that are not clearly defined in formal organizational regulations and rules and also recognized by organizational reward systems. This study was designed to figure out the relationship between political self-efficacy and OCB, and the moderating effect of psychological capital on this relationship. Hypotheses were proposed based on perspectives of social cognitive theory and positive organizational behavior. The mechanism of the relationship between political self-efficacy and OCB was tested on the basis of 272 paired data collected from employees and their direct supervisors. Political self-efficacy was measured by the cumulative percentage differences of both the perceptions of their own political traits and organizational politics. The perception of political traits was measured by 16 items adopted from Machiavellianism scale developed by Dahling, Whitaker and Levy (2009), and the perception of organizational politics was measured by 9 items adjusted from Vigoda (2001). Psychological capital was measured by 24 items adopted from Luthans, Youssef, and Avolio (2006). OCB was measured by OCBI adopted from Williams and Anderson (1991) with 6 items in each scale. In order to avoid the common method variance problem in this study, two structured questionnaires, employee questionnaire and direct supervisor questionnaire, were employed as the measuring instruments. In employee questionnaire, perception of political traits, perception of organizational politics and psychological capital were included; and then OCB was evaluated the OCBI included in direct supervisor questionnaire. Results indicate that political self-efficacy had reversed U shape impact on OCB that both high and low levels of political self-efficacy are associated with low OCB, and the moderate level of political self-efficacy leads to the highest OCB. This finding not only extends and fulfills the organizational politics literatures, but also reveals practical implications for managers that they should help employees to maintain a moderate level of political self-efficacy to promote organizational effectiveness. Results also show that in addition to the positive effect on OCB, psychological capital plays as a conditional variable and moderates the reversed U shape relationship between political self-efficacy and OCB such that higher psychological capital weakens the tendency of reversed U shape impact of political self-efficacy on OCB which means that psychological capital acts as a substitute of political self-efficacy. In application, this finding suggest that managers should take psychological capital intervention as the moderating means to reduce the negative impact tendency of high self-efficacy on OCB. Moreover, theoretical contributions and shortcomings of this study are also discussed.