The variations and thereafter the process of changes of a particle '-sa', which is supposed as cognate forms, was discussed in the integrated method of dialect research and comparative study. The main topic was the semantic-functional analysis of '-sa' at the postpositional distribution. In Korean dialects the particle '-sa' has settled into grammatically-stable stage, that is, a postposition. In Japanese dialects, the particle '-sa' has shown the expansion of the extension, that is, the expansion of meaning and usages. In conclusion, we could find the both directions of grammaticalization and narrowing down of a grammatical particle.