Abstract: The origins of contact fatigue failure of involute cylindrical spur gears were investigated via surface failure analysis combined with finite element stress analysis. Results show that greater relative sliding, maximum exhausted frictional work and exposed to frictional stress greater than that during the second double tooth meshing,contributed to the densely covered surface pitting of the dedendum surface from the gear-in point to the lowest point of single tooth contact. Heavy spalling or subsurface initiated pitting in the dedendum surface close to the pitch line was resulted from the maximum Hertzian contact shear stress of maximum distance below surface and the impact as a result of disengagement of the foregoing meshing tooth. Shallower spalling in the surface immediately after the gear-in point was related to the impact of gear-in,shallower and relatively larger subsurface maximum shear stress and maximum relative sliding,and was promoted by the surface pitting in this region.