This research was conducted in order to investigate the educational effect of English-medium
instruction by comparing the learning outcome achieved from two courses(Introduction to
Philosophy) taught by the same instructor in English and Korean respectively. The study, using
a quasi-experimental design, based on a sample of 232 students enrolling in a private university
in Korea. The pre- and post test comparison of the two groups' academic achievement
controlling gender and the number of finished semesters, revealed that the English group
achieved the same level of learning outcome as the Korean group. However, another group
comparison assessing epistemological beliefs indicated that, while six subscales of epistemological
beliefs out of seven did not show statistically significant change over time across language
groups, the subscale of regulated learning of the English group showed a significantly negative
change compared to its counterpart. These results imply that, while an English-medium course
systematically organized and taught by an instructor with sufficient English fluency can achieve
the same level of learning outcome as Korean-medium course, nevertheless, EMIs' potential
negative effects on students' learning process should be still heeded and further investigated..