This paper discusses the prosodic change from Middle Korean to Modern Korean. While Middle Korean was a tone language, Modern Korean has a length opposition without a tonal opposition. About this prosodic change in Korean it has been said that long rowels in Modern Korean have come from the syllables with a rising tone in Middle Korean. But there are many exceptional cases : From the syllables with a rising tone short vowels, not long vowels, have developed and long vowels have come from the syllables without a rising tone. These exceptions can be properly explained under consideration of four factors: tone, initial strengthening, syllable cut and compensatory lengthening. Syllables with a rising tone in Middle Korean have a long vowel in Modern Korean only in the initial position in a word or a breath group. It results from the initial strengthening which is a universal phonological phenomenon. It is argued in this paper that the phonological feature of Ipsong in Middle Korean can be interpreted in terms of the syllable cuts are appropriately described and analysed within the nuclear phonology. Ipsong is not a language-specific characteristic, but a representation of an universal prosodic feature. it is interpreted as an abrupt syllable cut and plays an important role in the reorganization of the phonological system from the tonal opposition in Middle Korean to the length opposition in Modern Korean. From syllables with Ipsong short vowels, not long vowels, have developed despite their rising tones, and if long vowels have come from the syllables without a rising tone, it is generally the case without Ipsong. It seems that a compensatory lengthening at the phonetical level took place in Middle Korean. But long vowels in Modern Korean have come from the phonological compensatory lengthening that happened after the phonological change from tone vowel length.