Abstract: The effect of tribenyl phosphate (TCP) and dibutyl phosphite (DBP) as additives on the tribological behaviors of rapeseed oil was studied with a four ball machine. The worn surfaces were analyzed by means of X ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). It has been found that both TCP and DBP as additives increase the load carrying capacity and antiwear ability of the rapeseed oil base stock. However, they are ineffective in improving the friction reducing behavior of rapeseed oil. Especially, TCP with higher concentration in the base stock registers considerably increased friction coefficient. XPS analytical results of the worn steel surfaces indicate that tribochemical reactions were involved for steel steel frictional pair lubricated with DBP rapeseed oil and TCP rapeseed oil. This contributes to increase the antiwear ability and load carrying capacity of rapeseed oil.