This study aims to clarify the changing processes of the on-glide j-diphthongs in Korean, especially in the post-consonantal. In order to achieve the aim, we examine the pre-forms with an on-glide diphthong and their contemporary forms with a monophthong, and then elucidate their diachrony through comparing the pre-forms and their equivalent forms in dialects, having regard for its vowel system and diphthong constraints. As a result, we identified that the change of an on-glide diphthong are related to the change of a vowel system and diphthong constraint.