This study analyzed university and college graduates' income gap in South Korea to investigate factors influencing such disparities. Specifically, this study focused on types of higher education institutions and academic disciplines among the many factors affecting post-graduation income differences, using a hierarchical linear model. According to the analysis, there were differences in post-graduation income of graduates based on types of higher education institutions they attended. There was an especially large income gap between graduates from 4-year universities located in Seoul and other types of institutions. However, the coefficient size of income differences between graduates from 4-year regional universities and 2-year junior colleges either decreased, when SES variables were controlled for. This result implies that income gap between 4-year regional universities and the other types of institutions can be explained by the differences in students' socioeconomic background. In addition, the effects of SES on graduates' income varied in academic disciplines. Based on these results, it can be said that the horizontal stratification of higher education is affected by the combination of institutional types and academic disciplines in Korea.