This study employed the concept of undermatching, which refers to entering a college in which the student is at a lower level than a school where he or she can attend, to explore the extent to which students are undermatched in South Korea and its association with socio-economic status. Using a nationally representative sample of KEEP:04(Korean Education & Employment Panel: 2005, 2008 cohorts), the results show that nearly half of students are undermatched, and it is more prevalent in the low socio-economic status group. Specifically, the strongest association between socio-economic and the likelihood of undermatching was found among the high achieving group of students. We suggests the policies to eliviate the problem of undermatching.