Abstract: Equal-base-circle bevel gears can be cut precisely in an economical way, which is of importance to large size bevel gears. Taking tooth trace modification into account, the equation of cutter center trace was derived. To carry out tooth profile modification, the modified cutter profile equation was determined for equal-base-circle bevel gear profiles. According to differential geometry theory, the primary curvature and primary direction and normal curvature in different directions of cutter surface were obtained. Based on the theory of conjugate tooth surface, the primary curvature, primary direction and normal unit vector of gear tooth surface were derived from the geometry structure of cutter surface and the special relative motion between the cutter and tooth blank, which can be used to predetermine the meshing properties. All of these lay a foundation for gearing analysis, modification and strength calculation. The modification design of equal-base-circle bevel gear, which is available to improve meshing quality, can be carried out easily by NC technique.