In the Sukhothai period, Thai language had various reference terms for Buddhist monks because of their particularly high status in the society. This research aims to study categories of reference terms for Buddhist Thai monks and their lexical... and semantic changes in the Sukhothai period in comparison with the Present. Data had been collected from 41 edited Sukhothai Inscriptions and analyzed through a framework of reference terms and rules of language change. This research has collected 74 reference