Abstract: The effect of S-type additive T321 and P-type additive P120 on the EP and AW abilities of rapeseed oil was investigated on a four-ball machine, and the synergistic effect between the two additives was also examined on the same friction and wear tester. Thus the p_B and p_D values of the rapeseed oils with or without the additive were determined according to the standard methods, while the corresponding wear scar diameters of the steel balls were measured on an eye-sight microscope. The wear scar of the steel ball was observed on a scanning electron microscope. It was found that both the T321 and P120 additives were able to greatly increase the load-carrying and extreme pressure and antiwear properties of the rapeseed oil. Specifically, the complex additive made of T321 and P120 with a proper composition was most effective to improve the comprehensive tribological behavior of the base stock, which was attributed to the synergistic effect between the two additives.