<p>Gear contact fatigue failure mechanism has become an important bottleneck problem to be solved by industry. Based on the modified layer gradient bearing effect of carburized gears, a coupled mathematical model for risk prediction of contact fatigue of gears with gradient loading was developed. The study demonstrates that the stress risk domain is a consequence of near surface-to-surface movement and the friction tangential load increment of the characteristic parameter. The cracks may form on the near surface and extend to pitting failure under conditions of adequate lubrication. And under inadequate lubrication, the surface stresses may increase, resulting in the formation of cracks and extension to micro-pitting. The material load-carrying parameters are enhanced through the application of surface hardness increments, thereby improving the load-carrying capacity of the gear contact. And gear contact fatigue failure is easily accelerated by normal load increments. The predictive model is in perfect agreement with the pitting and micro-pitting crack initiation mechanisms and failure life characteristics demonstrated in actual gear running class loading tests.</p>