Abstract: Several S-containing borate esters and one borate ester based on boric acid and n-octanol were synthesized. The antiwear and friction-reducing properties of the synthetic compounds as additives in rapeseed oil were examined with a four-ball machine. The relationship between the additive structure and tribological properties was explored, while the lubrication mechanisms of the additives were investigated as well. Thus the worn surfaces of the steel ball lubricated with the additive-containing rapeseed oil were analyzed by means of X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). It has been found that S-containing borate esters as additives in rapeseed oil at proper concentrations are effective in improving the antiwear and friction-reducing behavior of the rapeseed oil. The polar rapeseed oil experienced competitive adsorption and tribochemical reactions with the additive on the rubbing steel surface. However, the tribological behavior of the synthetic additives does not depend on the number of S atoms in the molecule. This implies that the tribological behavior of the additive might depend on the ratio of B to S in the additive molecule. This was confirmed by the result that the rapeseed oil containing additives having the same element composition and similar molecular structures exhibit similar tribological behavior. Therefore, it is inferred that there exists a synergistic effect between S and B atoms in the additive molecules.