Involute helical gears mesh based on the intersections of involute helicoids. However, spiral involutes on the tooth surface do not participate in meshing directly. A new type of gear drive, the spiral involute gear drive is proposed that works on contact of spiral involutes. The generation of tooth profile is introduced in detail. Through relative-stagnation method spiral involutes prove to have conjugation characteristics. To testify whether the transmission ratio of cylindrical spiral involute gears is constant, simulation is implemented in commercial codes ADAMS based on solid models of a pair of spiral involute gears. The computed results show that this novel gear drive can achieve a constant transmission ratio. Due to transmission with uniform velocity, cylindrical spiral involute gears can be used in transmission between intersecting axes, so that generating milling and generating grinding can be achieved.