The purpose of this paper is to analyze vowel harmony in Middle Korean within the framework of Government phonology. we establish three licensing constraints for the nuclear expressions of Middle Korean: ⅰ) I˚ must be a head and cannot license other elements; ⅱ) U˚ cannot be a head without an operator, ⅲ) the number of elements within a nucleus is maximally two. Based on these constraints, Middle Korean nucleus expressions can be represented as follows: ‘ㅣ’/i/(I˚), ‘`’/∧/(U˚.A$^{+}$), ‘ㅡ’/ł/(v˚), ‘ㅏ’/a/(A$^{+}$), ‘ㅓ’/∂/i/, middle Korean nuclear expressions can be divided into two set, a set of v˚-headed nuclei (ㅡ, ㅓ, ㅜ :traditionally called ‘dark vowels’ ) and a set of $A^{+}$U˚-headed nuclei (、, ㅏ, ㅗ : traditionally called ‘light vowels’). Vowel harmony in Middle Korean has been treated as a directional harmony. Government Phonology explains directional harmony as spreading of an element from a domain-final nucleus (the initial nucleus in the case of Middle Korean vowel harmony) to all successive nuclei through government between nuclei. This assumption is, however, not sustainable because of the neutral vowel /i/’s appearance in the initial vowel of not only dark vowel words but also light vowel words. This paper proposes the notion of ‘dominator-licensing’. Dominator is an element in a suprasegmental tier. It licenses itself to nuclei within a dominating domain. When a nucleus is ’d-licensed (dominator-licensed)’, it takes the dominator as its head and its original head is switched into an operator. Dark vowel words in Middle Korean has a dominator v˚ in their suprasegmental tier. When it is associated to a nucleus, it lands as a head of the nucleus and its original head is changed into an operator. Its original operator, if any, is dissociated from the nucleus due to the licensing constraint that the number of elements in a segment cannot exceed more than two. This explains the alternation of ‘ㅏ’(a)/ ‘ㅓ’(∂)/ and ‘ㅗ’(o)/ ‘ㅜ’(u). When the nuclear expression ‘`’(∧) is d-licensed by v˚, however, it drops not only its original operator but also its original head. It is attributed to the instability of inner structure of /∧/ in Middle Korean, which is supported by the fact that the vowel /∧/ in Middle Korean has been gradually merged into other vowels later. Unlike other vowels, the neutral in that the nucleus /i/ cannot be associated to the dominator v˚ due to the licensing constraint that the element I˚ must be a head and it cannot license an operator.