More colleges in non-English-speaking countries are adopting an English-medium instruction (EMI) policy. Korea Advanced Institute of Science and Technology has implemented an EMI policy since 2007. The purpose of this study is to investigate the effectiveness of EMI and to make suggestions for more effective EMI for undergraduate students at a Korean engineering school. A sociology course offered in the fall of 2012 was chosen for the study. The course was offered in two sections, one with Korean-medium instruction (KMI) and the other with EMI, by the same instructor. English tests were given to measure students’ English ability. Questionnaire and interview surveys were conducted. In addition, course evaluations, test scores, and grades were compared between the EMI and KMI groups. The study results were as follows: the KMI students showed higher levels of satisfaction than the EMI students; the EMI students’ grades showed a high correlation with their English test scores, and among the linguistic skills, the students’ listening skills showed the highest correlation with their grades. The majority of the surveyed students answered that the school should continue offering EMI classes, but only to the students who prefer, taught only by the professors who wish to teach such classes. These results prove the arguments by Johnson and Swain (1994), Burger, Wesche, and Migneron (1997) and Marsh, Hau, and Kong (2000) that language proficiency is crucial for successful late immersion classes and that when students are placed in EMI classes without consideration of English proficiency, their satisfaction levels are low.