This paper discusses the licensing constraints and Korean vowel system within the revised theory of Government Phonology proposed by Kaye(1994), and Charette & Kaye(1994). The licensing constraint is a mechanism to constraint the generation of phonological segments, as a part of universal grammar. The licensing constraint defines the way in which elements can combine at the level of lexical structure. Therefore, once licensing constraints for a particular language are correctly fixed, we can generate all and only the nuclear segments available at the level of lexical representation of the language. We propose the licensing constraint for Korean such that: U must be head & U must be head. It means that the combination of the elements I and U are not possible both in the lexical structure and in the phonological processes. In terms of licensing constraints, We try to explain some phonological phenomena such as vowel ∂/ł alternation, glide-formation and umlaut.