This study investigates the declination of vowel harmony in Korean from the 15th century to the 18th century. Between the 15th and the 18th century, restructuring of the vowel system resulting from vowel shift brought about diachronic declination of palatal vowel harmony between front and back vowels. Since the 15th century, vowel harmony in Korean has been in steady decline. Noting this, the present paper examines the frequencies of vowel harmony from the 15th to the 18th century. To do so, it analyzes a historical Korean corpus from the perspective of Information Theory, measuring the quantitative analysis index of vowel harmony by the information-theoretic notions of ‘positive logarithm’ and ‘mutual information.’ Mutual information (MI) between the vowels in autosegmental tiers leads to two findings about the declination of vowel harmony in Korean. Firstly, there were two sharp declines of vowel harmony in the historical Korean corpus. There was first a declination between the 15th and 16th centuries, and the second declination occurred between the 17th and 18th centuries. Secondly, the prohibition of non-harmonic vowels is more effective than the preference for harmonic vowels in Korean vowel harmony. (Keimyung University)