Extended tooth contact engagement with reduced tooth stress is achieved by first defining the tooth face profile of one of the gears, namely the circular spline, by a circular arc segment. Next, the tooth face profile of the other gear, namely the flexspline, is defined by a curve shape that allows several teeth to remain in contact while the wave generator is rotated. Furthermore, the transition region between tooth flank and tooth face is made continuous, and thereby smoothed out, by using a straight line segment for this portion of the flexspline tooth profile. The aforesaid curve shape that allows several teeth to remain in contact is determined by establishing the movement locus of a point on the flexspline tooth in relationship to a point of contact between the face profile of the flexspline teeth and face profile of the circular spline teeth and subtracting the tangential component shift due to tooth inclination as the wave generator is rotated.