This study examines the Korean diphthong 'ㅢ' and proposes a new underlying form to address existing issues. The current analysis of 'ㅢ' as a glide-vowel sequence has several problems: lack of diachronic evidence, contradiction with the cross-linguistic definition of diphthongs as single phonemes, and difficulty explaining 'ㅢ'’s phonetic variants. As an alternative, this study suggests analyzing 'ㅢ' as /ɯi/, a single phoneme combining [ɯ] and [i]. This approach aligns with the cross-linguistic definition of diphthongs, meets phonemic criteria, and explains the various phonetic forms of 'ㅢ'. This analysis emphasizes the universality of Korean phonology and provides a basis for comparative studies. Analysis of other diphthongs apart from ' ㅢ' will be left for future research.