Gear tooth contact analysis and a FZG gear test rig were applied to study and analyze gear contact fatigue in this paper. Under the range of operating conditions, experiments were performed in the test rig by using standard FZG type C spur gear test specimens which were made from 20MnCr5 gear steel. Test rig, spur gear test specimens, the properties of the gear material were also introduced. After the test, pitting formation was investigated using scanning electron microscopy. Two kinds of fatigue spallings caused by subsurface fatigue failure and surface fatigue failure were investigated. And energy spectral analysis for two kinds of fatigue positions were analyzed by the EDS detector, the materials of subsurface fatigue failure are the oxide $Al_2O_3$ and the carbide, and the black textures of surface fatigue failure are the oxide and the non-martensitic material. Through the mechanism analysis of this gear contact fatigue failure, it is founded that reducing the oxygen content in the material, can effectively reduce the oxide inclusions in the steel. It would provide a reliable reference study for 20MnCr5 carburized gear.