This study investigates aspects of vowel harmony in Late Middle Korean from the 15th century to the 16th century. Since the 10th century, restructuring of the vowel system resulting from vowel shift brought about the diachronic declination of palatal vowel harmony between front and back vowels. Since the 15th century, vowel harmony in Korean has been in a steady decline. To examine the frequencies of vowel harmony from the 15th to the 16th century, I analyze a historical Korean corpus from the perspective of Information Theory, measuring the quantitative analysis index of vowel harmony by means of the information-theoretic notions of positive logarithm and mutual information. Mutual information (MI) between the vowels in autosegmental tiers leads to three findings regarding vowel harmony in Late Middle Korean. First, the preference for harmonic positive vowels in the Korean vowel system is more effective than the preference for harmonic negative vowels. Second, if the preceding vowel is a diphthong beginning with a /w/ glide, there is little effect of vowel harmony in Middle Korean. Finally, the vowel harmony between monophthongs is more regular than that between diphthongs or triphthongs. (Keimyung University, Assistant Professor)