The influence of hardness combination of gear pairs (gear ratio u=⃥1) on their surface durabilities was investigated by using a power circulating-type gear testing machine. The following results were obtained for the hardened and tempered gears (HB ≃340) : (1) In the case of hobbed gears with some difference in hardness, (i) the pitting area ratio of softer gear does not decrease, (ii) the tooth profile of softer gear degenerates, (iii) the influence of hardness combination of gear pairs having gear ratio u=⃥1 on their surface durabilities is more remarkable than that of gear pairs having gear ratio u=1. (2) In the ground gears, (i) the higher surface durability by the effect of hardness combination does not attain, (ii) when the tooth surface of harder gear is rougher than that of softer gear, many pits occur in the softer gear with repeated contact stress. Therefore, the influence of hardness combination on the pitting failure is much related to the tooth surface roughness of two mating gears.