Abstract: The comparative test of the contact fatigue life of 45# steel was performed with four lubricating oil samples whose mixture ratio are shown in Table 1. Table 1 The mixture ratio of four lubricating oil Samplex Mixture ratio (wt) A pure 20# machinery oil B dispersing agent : 20# machinery oil = 1: 100 C carbon powder : dispersing agent : 20# machinery oil=1: 1 : 100 D carbon powder : dispersing agent : 20# machinery oil = 2 : 1 :100 The results shown that under using C and D samples with carbon powder the contact fatigue life of 45# steel is 2.6 and 5.0 times than that of using A sample respectively. In addition, the mechanism of carbon powder was also investigated in this paper. The reason of increasing contact fatigue life by adding carbon powder are as follows: a. The viscosity of the lubricating oil samples increases with pressure increasing. The higher the carbon powder content, the quicker the increasing of rate. The pressure on contact point between test bar and ball can reach 2000MPa. The viscosity of samples with carbon powder is higher than that of samples without it. The thickness of oil film increases, therefore, contact fatigue life of testing piece prolongs. b. When the fatigue crack is formed on the contact surface the carbon powder in the sample can be infased into the crack, because of hard carbon particles the opening of crack can not be blocked although the steel ball roll it, so the high pressure in the crack can not be formed. Adding the carbon powder into lubricating oil can decrease the extending rate of crack and prolong the contact fatigue life of testing piece.