Abstract: Taking rapeseed oil as base constituent, the three lubricant additives, ie. sulfur-system T321 and T307 and phosphorus-system P120, were investigated separately on a 4-ball friction-wear tester with the intention of how they improve the wearability and extreme pressure characteristics of rapeseed oil. The results showed that they all are beneficial to reduce wear-out and improve the load capacity and resistance to extreme pressure of lubrication film. The SEDX (Spectral Energy Distribution X-ray) analysis of steel ball surfaces revealed that there are tribochemical reactions of the rapeseed oil containing any of the three additives during friction/wear process, thus forming a boundary lubrication film composed of triglyceride and others produced from the reacted additive to improve the wearability and load capacity.