An Optimality Theoretic Approach to Semi-vowel [j] Jo, Hak-haeng and Seo, Jeong-min(Chonsun Univ.) There have been three kinds of arguments about the status of semi-vowel [j] in Korean within the Korean syllable structure. The three theories, Nucleus Hypothesis, Onset Hypothesis, and Tautosyllabic Hypothesis, consider the semi-vowel [j] in Korean as part of nucleus, onset, and both respectively. Each theory, however, has provided conflicting accounts in reference to analogous evidence. Thus, in this paper we argue that the permission of semi-vowel [j] of Seoul dialect in Korean can be given a functionally grounded treatment by means of *ONSET/X subhierarchy(Smith, 2003: 3) that maintains its universally fixed ranking based on the sonority scale, if the *ONSET/X is defined to be sensitive to the moracity of the segments. And then according to Optimality Theory(Prince & Smolensky, 1993) we will analyse semi-vowel [j] of Seoul and Chonnam dialect in Korean as nucleus. And then the result will be extended to the semi-vowel [j] in English.