This research investigates Vowel Lengthening (VL), including Compensatory Lengthening (CL), in Slavic languages. Slavic languages inherited the linguistic properties from *Proto‐Indo‐European (*PIE), Proto‐Slavic, and Late Common Slavic (LCS) and developed these properties into their own dialectal variations. That is, each Slavic group adopted the Slavic universal properties and developed them into its own language‐specific properties. VL can occur either by phonemic factor or in phonological factor. The latter can be found in Russian where vowel length marking is not phonemic, but phonological, whereas the former can be found in Czech and BCS where length marking is purely phonemic and there is a contrast between long vowels and short vowels. VL in Slavic can be found in Czech, Polish, BCS etc., especially in the diagram of nouns. Czech with purely phonemic length is in contrast to Russian in the determination of vowel length. Polish has the systematic stress patterns where primary stress falls on penultimate syllable. As a part of VL, CL occurs under the following condition: in the structure of C1V1C2V2, the length of V1 becomes longer due to the loss of V2. Therefore, CL refers to “a set of phonological phenomena wherein the disappearance of one element of a representation is accompanied by a corresponding lengthening of another element” (Kavitskaya 2002:3). This kind of CL can be seen in Slavic linguistic history, especially in the process of diachronic development from LCS to modern Slavic languages. Reduced vowels in Old Church Slavic (OCS), whose properties are inherited from LCS and even *PIE, exhibit the full process of CL by dropping reduced vowels in weak positions or in the final position of words. Open‐syllable word structure of LCS tradition changed into closed‐syllable structure, whose structure is of CVCV  CV:C by dropping the final reduced vowel. This change made an influence on the linguistic change of each Slavic group and each Slavic language. CL is a context‐free process and subject to prosodic constraints. Moraic theory is suited for the formal analysis on the Slavic language specificity. In this respect, Slavic data demonstrates that moraic theory is well suited for Slavic CL and VL and also CL turned out to be a context‐free process even in Slavic.