Most research on corporate social responsibility(CSR) has primarily taken the perspectives of investors or customers, who are the external stakeholders of the company. The research focus is the effects of CSR activities on the company’s financial performance and customers’perceived brand image or their intention to buy. The neglected perspective, that of employees,however, is critical to understand how CSR affects company performance. The success of the company is critically mediated by employees who interpret the activity of corporate CSR and mobilize commitments and work motivation based on their interpretation. Despite the importance of employees in corporate CSR, there are few studies that empirically examine the effects of CSR through the lens of the employees. Furthermore, previous researchers who explored the effects of CSR taking the employees’ perspectives have not recognized the difference between external stakeholders and employees. Since employees as internal stakeholder can have relatively more access to information about the company intention for CSR than can external stakeholders, they derive different values and goals by interpreting this information and respond differently from investors or customers. The purpose of this study is to examine whether employees' perceptions of CSR legitimacy influence on their organizational citizenship behavior(OCB) and counterproductive work behavior (CWB). We conceptualizes legitimacy as two components: external legitimacy and internal legitimacy. When employees attribute the motive of CSR to firm's competitive advantage, they will interpret the firm's CSR as a window-dressing corporate behavior, which is explained by external legitimacy. In contrast, employees will interpret the firm's CSR to be based on the firm's authentic behavior when they attribute the motive of CSR to attain firms' mission or vision, which is consistent with the concept of internal legitimacy. It is predicted that employees will engage in more OCB and less CWB when they perceive that CSR reflects internal legitimacy. On the other hand, it is hypothesized that employees will exhibit more CWB and less OCB when they perceive that CSR reflects external legitimacy. In addition, it is proposed that employees' cynicism about CSR will mediate the relationships between employees' perceptions of CSR legitimacy and their OCB and CWB. We tested the hypotheses with the sample of 399 employees from 38 companies that represent the population of Dow Jones Sustainability Indexes(DJSI) Korea 2009 survey. The results of LISREL analysis confirmed that all the predictions of this study were supported. The internal legitimacy increases organizational citizenship behavior and decreases the counterproductive work behavior through dampening the perceptions of cynicism on CSR. In contrast, the external legitimacy decreases organizational citizenship behavior and increases the counterproductive work behavior by intensifying the perceptions of cynicism on CSR. We indicated the effects reflected the janus effects, because legitimacy play two distinct roles on corporate CSR. We highlight that CSR enhances OCB and reduce counterproductive work behavior when employees perceive the authentic motive with internal legitimacy. However, it shows a ugly face when employees conceive it as reflecting purely instrumental motives with external legitimacy.