There were severe pitting corrosions on the surface of transmission gears in diesels. The cause of metal fatigue pitting of gear surface is analyzed in the paper, which based on the contact fatigue experiment of 18Cr2Ni4WA carburizing and quenching gears used in the diesel engine. Causes of pitting on harden tooth surface are studied from three aspects: gear surface residual stress field, oil film pressure and carburizing thickness, which according to the morphology of test gear fatigue fracture and the surface hardening properties of carburizing and quenching gears. The reasons of experimental gears' contact fatigue pitting were analyzed and conclusions were as follows: The depth of the diffusion layer, surface carbon concentration and the carbon concentration gradient of layer were the key to improve the ability to resist wear surface fatigue pitting.