Abstract: By using statistical technique of experiment and treating data, it is intended to investigate the effet of combined stresses on contact fatigue. The superimposed tensile bending stress markedly shortens the process of fatigue. But the effect of compressive stress is dependent on its magnitude. A small superimposed compressive bending stress can increrease the fatigue life, while the large compressive bending stress decreases it. There is critical compressive bending stress at which the fatigue life becomes maximum. According to the experimental results, the author attempts to analyze the availability of some hypotheses of decisive stress for estimation of contact fatigue, and indicates that taking the maximum surface shear stress as the decisive stress is more suitable than others. It seems that the initiation of combined stressed contact fatigue takes place at surface.