This study examined what the demotivating factors of Korean high school students are in learning English, and whether there exist any differences in demotivating factors by proficiency and gender. The results revealed that the participants’ demotivation mainly came from the difficulty of learning English, followed by grades and test scores. All the demotivating factors showed significant differences when considering the L2 proficiency, suggesting the need of level-based instruction. Furthermore, the differences between the high and the low proficiency groups were very large considering the calculated effect size. On the other hand, the differences between the boys and the girls were not that evident. Other demotivating factors the participants pointed out were non-practicality, lack of knowledge, school system, and teachers’ overexpectation and overestimation of learners. Pedagogical implications and suggestions are also made based on the findings.