Two groups of spur gears manufactured from two different materials and heat treatment were endurance tested for surface fatigue life. One group was manufactured from AISI 1552 and was finished ground then dual frequency contour hardened and the second was CEVM AISI 9310 carburized hardened then finished ground. The gear pitch diameter was 8.89 cm (3.5 in.). Test conditions were at maximum Hertz stress of 1.71 GPa (248 ksi) and a speed of 10 000 RPM. The lubricant used for the tests was a synthetic paraffinic oil with an additive package. The results showed that surface fatigue life of the contour hardened AISI 1552 was 1.7 times that of AISI 9310 gears.