Endurance test of carburized helical gears designed to realize extremely high load carrying capacity is carried out, where gears are designed optimumly against common gear failures such as tooth breakage, pitting and scuffing of tooth flank. The test showed some kinds of tooth failures caused by contact of tip edge and tooth side edge due to small amount of profile form and lead crowning' modification. Very small chipping failure occurs at side edge of tooth, although corner chamfer is given there. Chipping induces some subsurface microcracks and they propagate to result in tooth breakage. High contact pressure and highly stressed condition near tooth side edge, where the material is overcarburized and fragile, are considered to be major reasons of such chipping of tooth side edge. Corner rounding of side edge of tooth has an effect to reduce such chipping, compared with corner chamfer, and prevents tooth breakage due to crack propagation from chipping.