In order to discuss theoretically the effects of surface roughness on gearing contact fatigue stress,a mixed Elastohydrodynamic Lubrication（ EHL） model for heavy-duty gear transmission was established based on the transverse roughness features simulated by the fractal function,which were superimposed to the film equation. Applied the multi-grid method,40 groups of numerical calculations were completed. The results show that the average film thickness is almost linearly increased with an increment of the roughness value. But it is not simply the case that the less the roughness value,the better. When the roughness is less than a critical value,however,increasing roughness value is found to reduce the maximum principal shear stress on pinion subsurface. At the end,the quantitative relation between the maximum principal shear stress and the roughness is acquired based on the regression analysis of the calculation results.