Abstract: Based on the random occurrences of micro-pitting and thermal-scuffing in the engineering application, the damage mechanisms of these two failure modes and the laws related to their transition were analyzed in this study. A concept of the competitive mechanism between micro-pitting and thermal-scuffing failure of the high-torque and wide-speed-range gears was proposed. On the basis of the numerical methods of calculating the film thickness and flash temperature on the meshing faces, a combined evaluation criterion of strength check was established, and its rationality and validity was indicated by the experimental results. Finally, the bearing capacity of 18CrNiMo7-6 and plain mineral oil against scuffing and pitting damage was determined. Meanwhile, in the combination of numerical calculation and experimental tests, the ultimate contact temperature of thermal-scuffing was put out. The minimum safety factor in the design of anti-pitting was also proposed through the comparison of oil film thickness and surface roughness. Based on the conclusions, it is hoped to provide an analysis method for the designing of gears with high power density and wide rotation speed range.