Abstract: The effect of n alcohols and fatty acids as additives on the antiwear and extreme pressure properties of rapeseed oil in sliding of steel against steel as investigated using a four ball machine. The worn steel surfaces were analyzed by means of scanning electron microscopy (SEM) and X ray photoelectron spectroscopy (XPS). It has been found that n alcohol as additive in rapeseed oil has little effect on wear resistance and load carrying capacity, while fatty acid increases the antiwear ability but decreases the load carrying capacity considerably. This is closely related with the characteristics of rapeseed oil and the polarity of the base stock and additives. The XPS analysis of the worn steel surfaces indicates that tribochemical reactions were involved in the friction process and complex lubricating films were formed on the steel surface. The different characteristics of the lubricating films govern the different tribological behaviors of the additive containing rapeseed oil.