Recent reseach in the field of medical information systems has paid much attention to an ontology based medical terminology system to support clinical study and effective information search. This study aims to conduct research for further application or construction of ontology systems in Korea. This research reviews the theory of concept modeling and ontology. and analyses 4 cases of conceptual modeling of medical tenrminologies by ontology, The findings ot this study display these specific characteristics in medscal ontologies: (UHe standardization of terrrinology on MeSH. (2) The conceptualization of tenninology on UMLS. (l) ane. (2) are showed as untormal ontologies. (3) The theory of ontology integration in ON9, (4) The reference model of medical knowledge with formalization in GALEN. (3) and (4) are showed as formal ontologies. The application and construction of ontology should be differentiated according to the level of the proposed system, and then this analysis will provide useful information for the researcher and developer of the system.