International student mobility can provide important grounds for understanding the nature of knowledge network by revealing how the central and peripheral countries of knowledge and research are connected and transformed. Given the importance of such understanding, this study examined international student mobility by degrees and fields of subject using social network analysis. The analysis was based on the 2016 student mobility data provided by OECD and UNESCO. From the analysis, this study found that for all degrees, international student mobility has concentrated on USA, UK, Canada, Germany, and Australia, which have been traditionally central countries of international student mobility. On the other hand, Asian and Latin American countries still had a high level of student outflows. Also, for the higher degrees, international student mobility tended to flow to America and Europe, which include countries of high inflow centrality, rather than to various countries. Such concentration on a small number of countries of high centrality was also found for subject fields, particularly for Ph.D in science and engineering. The skewed flow to a few developed countries along with the worldwide increase of international student mobility suggests that imbalance of the global knowledge network has been aggravated. We discussed the implications of such imbalance with regard to higher education.