The government of Taiwan, aiming to lift English up to join Chinese as the 2nd official language, has declared that it will be officially bilingual by 2030. Projects on English-Medium-Instructions (EMI) have been spreading out and practiced in classrooms, and colleges in Taiwan are no exception. This study investigated the impact of EMI on two groups of college English majors taking a sophomore-level Introduction to Linguistics course, taught by a native speaker instructor and a Taiwanese instructor during an 18-week semester. Quantitative data were collected by 3 questionnaires, examining if the students’ engagement and self-perceived language skills had changed, and further investigating their attitudes toward EMI. Qualitative data consisted of semi-structured interviews with the students and the 2 instructors. The findings showed that: (1) The EMI course conducted by the native speaker instructor was more successful in strengthening the students’ learning behaviors and enhancing in-class interactions; (2) The EMI course taught by the Taiwanese instructor leveled up the students’ enthusiasm in embracing EMI and the students tended to feel more at ease with the Taiwanese instructor because the instructor had a better understanding in choosing materials and designing class activities tailored to meet the students’ English fluency. (3) Both student groups indicated that EMI had helped most in improving their listening comprehension, wiliness to communicate, and concentration to stay focused in class. This study calls for a bigger scale study to examine the impact of EMI on EFL learners in colleges of Taiwan, and possibly at schools of all levels.