In this paper, we estimate human capital externality within the firm and identify its differential effects, exploiting two-way fixed effect model. Our results show significantly 1.2% external effects within the firm. This result means that a worker within the firm that has high level of workers' average years of schooling has wage premium. This result suggests that the social returns to education from human capital externality are strongly positive, implying that the effect of education should be evaluated by positive human capital externality as well as private return to education not to underestimate the effect of education. We also estimate the differential effect of human capital externality with regard to personal and company characters. Human capital externality has differential effects by gender, marital status, occupational type, and incentive type. These results emphasize that a company should establish proper strategies in order to maximize the benefit of human capital externality.