The purpose of the present study was to examine the effects of grit at work. Especially, this study investigated the effects of grit, known as a personal success factor, on organizational citizenship behavior (OCB) which is voluntary behavior for the organization and colleagues. In addition, we focused on the mediation effect of job positive affect and occupational self-efficacy on the relationship between grit and OCB. Results from an online survey undertaken with Korean workers showed that grit had a positive effect on OCB when conscientiousness was controlled for. Moreover, job positive affect mediated the relationship between grit and OCB. On the other hand, occupational self-efficacy did not mediate the relationship. We discussed the implications of these results, study limitations, and practical suggestions for future research.