Abstract: Aim To unify the design methods of hypoid gears and spiral bevel gears. Methods The geometric calculation of these two kinds of bevel gears was analyzed and two coefficents of offset angle were defined. Many instances of these two bevel gears were checked. Results Some basic equations and the methods of evaluation were given, which can be used not only to hypoid gears but also to spiral bevel gears. Conclusion The unified method is helpful to the standardization and seriation of bevel gears and to CAD technique.