While rolling contact fatigue life of steels has been much improved by the improvement of cleanliness in steels due to progress in steel-making techniques, the demand for longer bearing fatigue life is ever-increasing. It will be necessary to find the best chemical composition and the best heat treatment conditions to further increase bearing fatigue life-Since bearing fatigue life also depends on the running conditions under which the bearing is used, it is also necessary to use material which meets the running conditions best.Carburizing steels generally have longer rolling contact fatigue life than through hardening steels. However, under debris contaminated lubrication such as in automobile transmissions, there were some cases where carburized bearings did not show such longer fatigue life. This paper describes test results of the investigations on the influences of various material factors such as chemical compositions, amount of retained austenite, and case depth of carburized tapered roller bearings on bearing fatigue life, especially under debris contaminated lubrication.In this experiment, it was revealed that a clear correlation exists between bearing fatigue life and core hardness. By making core hardness higher than usual (over HRC 50), the bearing fatigue life of carburized tapered roller bearings greatly increases, not only under debris contaminated lubrication but also under heavy loading conditions.