This study examines whether employees' legitimacy perceptions of corporate social responsibility (CSR) affect their organizational citizenship behavior (OCB). It also investigates whether CEO's visionary leadership can moderate this causal relationship. CSR legitimacy is defined in the current study as employees' personal beliefs about the appropriateness of corporate CSR activities. In fact, employees evaluate the appropriateness of CSR activity based on its consistency with corporate philosophy (e.g. corporate mission, vision, and values) which functions as employees' referencial belief structure. If CSR activity is perceived as one of firm's effort to fulfill its mission, vision, and values, which means that espoused theory and theory-in-use of CSR activity are congruent, employees will consider firm's CSR activity as legitimate. If, however, employees think that CSR activity is not congruent with firm's mission, vision, and values, which means that espoused theory and theory-in-use of CSR activity are inconsistent, they will perceive that CSR activity of their firm is not legitimate. In the current study, we propose that employees who perceive that the CSR activity of their firm is legitimate are more likely to engage in OCB. In addition, we hypothesize that CEO's visionary leadership can strengthen the positive effect of employees' perception of CSR legitimacy on their OCB. We tested these hypotheses with the sample of 383 employees from 32 companies listed on DJSI (Dow Jones Sustainability Index) Korea 2009. We employed the HLM (hierarchical linear modeling) program to decompose the multi-level random effects. We found that CSR legitimacy perceptions of employees increase employees' OCB and that CEO's visionary leadership moderates this relationship. We discussed implications of these findings in more detail.