Abstract: Four-ball tests were performed for rapeseed oil (RSO) containing the sulfurized butylene (T321) and the overbased calcium alkyl sulfonate (T109) to investigate the tribological properties of T321, T109, and their synergistic effect. The results show that the addition of T109 can retain the extreme pressure properties of rapeseed oil containing T321, improve its anti-wear performance and prolong its working life. The element compositions on the worm surface of the tested steel balls were detected by EDS technique and the synergistic effect of T109 and T321 was analyzed. It is found that in comparison with 10s of extreme pressure test, except of obvious carbon, calcium deposition, no large difference was found for other elements in 30 min test. Thus the improved anti-wear ability of the lubricant with two additives seems to be caused by the formation of inorganic boundary film containg calcium and carbon on the friction surface. While the extreme pressure performance fails to ascend is due to the extreme pressure testing time is too short to make CaCO3 participate in the friction reaction.