When designing a gerotor, designers determine basic dimensions of a gerotor with transmitted power considering strength, interference and so. on. But, designers can not easily obtain the tooth profile generated by dimensions as well as the geometry bob for cutting the tooth profile. In order to resolve these problems, an automatic design sys-tem creating not only the solid model of a gerotor but also that of the generating hob using the design parameters of dimen-sions is developed. Through the developed system, designers can improve the efficiency of design and satisfy the variable requirements of design as well. In this research, the three-dimensional solid models for gerotors are generated considering the design parameters. Besides, those for generating hob regarding the design parameters of hob is created automatically.