With the development of semantic web technologies in information retrieval area, the necessity for thesaurus is recently increasing along with internet lexicons. A thesaurus is the combination of classification and a lexicon, and is the topic map of knowledge structure expressing relations among concepts(terms) subject to human knowledge activities such as learning and research using formally organized and controlled index terms for clarifying the context of superordinate and subordinate concepts. However, although thesaurus are regarded as essential tools for controlling and standardizing terms and searching and processing information efficiently, we do not have a Korean thesaurus for geology. To build a thesaurus, we need standardized and well-defined guidelines. The standardized guidelines enable efficient information management and help information users use correct information easily and conveniently. The present study purposed to build a thesaurus system with terms used in geology. For this, First, we surveyed related works for standardizing geological terms in Korea and other countries. Second, we defined geological topics in 15 areas and prepared a classification system(draft) for each topic. Third, based on the geological thesaurus classification system, we created the specification of geological thesaurus. Lastly, we designed and implemented an internet-based geological thesaurus system using the specification.