Abstract: Three S containing borate esters (abridged as BS1, BS2 and BS3) were synthesized. The antiwear and friction reducing properties of BS2, commercial additive tricresyl phosphate (coded as TCP) and the mixture of BS2 and TCP of a mass fraction 1:1 as additives in rapeseed oil were examined with four ball machines. The relationship between additive structure, composition and tribological properties was explored, while the lubrication mechanisms of the additives 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 behavior and BS2 exhibits better antiwear ability than BS1 and BS3. All the additives BS2, TCP and BS2P can improve the load carrying capacity and antiwear ability of rapeseed oil within the tested concentration but are not always effective in improving the friction reducing behavior. The composite additive BS2P shows better tribological behavior than BS2 and TCP. This is attributed to the synergistic effect between BS2 and TCP. The XPS analytical results of the worn steel surfaces indicate that tribochemical reactions were involved for steel steel frictional pair lubricated with BS2P/RO, with the formation of a boundary lubricating and protecting film composed of triester of rapeseed oil and tribochemical reaction products of the lubricants. This contributes to improve the tribological properties of the rapeseed oil base stock.