As technical intellectual work now has become prolific and various, rapid evolution of technical terms needs to collect and manage them systematically and efficiently. The purpose of this study is to suggest the microstructure for systematic definitions of definiendum in terminological dictionaries. This study reveals specific properties or differences of terminography on the basis of terminology and lexicography, analyzes problems of microstructure in a current terminological dictionary, and, based upon the theoretical and practical analysis, attempts to derive structural and methodical suggestions for microstructure of terminological dictionaries.