This thesis is aiming to analyze phonological phenomena of Korean vowel harmony in a word. The theoretical ground for the vowel harmony is presented in terms of the Element theory. Based on this, the internal structure of vowels of Korean is presented. Vowel harmony in a word occurs mainly on ideophones and color words. And these phenomena are caused by the mechanism of generating a new word reflecting the differentiation of the word nuance through the change of the vowel. The underlying forms of vowel harmony are /ə/ and /u/. And the harmony feature is the element A. The element A is first combined with first nucleus, which changes its vowel value and then this vowel harmony feature is spreaded onto other followed nucleus. This spreading changes vowel of nucleus by strengthening element composition. (Hankuk University of Foreign Studies)