Abstract: In the research of spur gears with high speed, the theory of Johnson's load-sharing concept was employed to analyze the influence of surface roughness and thermal effect on lubricating properties. The distribution of the lubricant film thickness, the shear stress, and the friction coefficient along line of action (LoA) were revealed. The solution result shows that the contact of asperities is still unavoidable even under high speed, the lubricant film thickness decreases obviously when thermal effect is considered, particularly at the point where the meshing is finished. Moreover, the shear stress of the film changes tremendously at the transition point where the engaged teeth shift from two pairs to one pair or on the contrary, and the friction coefficient has step change. The point where sliding to rolling ratio equals zero impels the shear stress to decrease instantaneously down to zero, and the friction coefficient fluctuates simultaneously.