The theory of generate-line method has been discussed in this study. It is a new theory of manufacturing spiral bevel gears of which tooth surfaces are formed by exact spherical involutes. The tooth surface of spiral bevel gears is obtained by the pure-rolling motion between the base cone and the great circular plane of the fundamental sphere. Based on the cutting motions the equations to describe tooth surfaces have been derived by using theory of gearing, and the equation of meshing of spiral bevel gears with spherical involutes tooth surface is obtained in the text. This study can provide some fundamentals for manufacturing and contact analysis of spherical involutes spiral bevel gears.