Abstract: The ionic liquid of 1 methyl 3 butylimidazolium hexafluorophosphate was prepared and evaluated as a kind of promising lubricant for a steel/steel contact. Thus the friciton and wear behavior of the steel/steel system lubricated with the ionic liquid was examined on an Optimol SRV oscillating friction and wear tester, by comparing with that under the lubrication of phosphazene X 1P and perfluoropolyether as the lubricants. The elemental composition and chemical nature of the boundary films generated on the steel worn surface were analyzed with a scanning electron microscope equipped with a Kevex energy dispersive X ray analyzer attachment and an X ray photoelectron spectroscope. As the results, the synthetic ionic liquid showed excellent friction reducing and antiwear behavior and was superior to the phosphazene X 1P and perfluoropolyether in terms of friction reduction ability and load carrying capacity. A boundary lubricating film of ionic liquid was formed on the steel worn surface by physical adsorption, while chemical reaction of the ionic liquid with the steel surface was involved at an elevated temperature to generate a boundary lubricating film containing FePO 4 and FeF 2. This contributed to reducing the friction and wear.