This research has two aims. The one is to describe tonal change which appears when phonological operations, such as vowel deletion, vowel assimilation and semi-vocalization, occur. Another aim is to elucidate the change of tonological systems. The tonal change can`t be viewed as compensatory lengthening caused by non-syllabification. The change implies an instability of the tonological system and change of tone arrangement. Above all, there are several evidence which shows that the tone change can`t be viwed as compensatory lengthening: i) the tone can function as distinctive feature when the verb conjugates, ii) the juxtaposition of tones can be seen as a fluid rising tone. and iii) when tones are juxtaposed in a syllable, the syllable is lengthened after the second syllable. The evidence which shows that the tone change was caused by instability of tonological system and the change of tone arrangement are as follows. First, a polysyllabic verb stem with a departing tonal pattern becomes the one with a fluid rising tone due to non-syllabification. Secondly, in the Middle Korean, riging tone didn`t appear although semi-vocalization applied in front of the ending ‘-a/eX’, although riging tone is shown in the same position in Jeongseon dialect.