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Two kinds of thiophosphate, tri-n-octyl thiophosphate and tri-n-octyl tetrathiophosphate, were synthesized. The tribological behavior of the synthetic compounds and tricresyl phosphate as additives in rapeseed oil for steel-steel frictional pair was investigated 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 spectrometry and scanning electron microscopy, and the elemental compositions and distributions in the worn steel surfaces were observed and determined with an EPMA-810Q electron probe micro-analyzer. It has been found that the synthetic thiophosphates as additives in rapeseed oil at proper concentrations show better tribological properties than tricresyl phosphate. Tribochemical reactions were involved for steel–steel frictional pair lubricated with the rapeseed oil containing thiophosphates, with the formation of a boundary lubricating and protecting film composed of glyceride of rapeseed oil and tribochemical reaction products of the lubricants. This contributes to improve the tribological properties of the rapeseed oil base stock.