Abstract: Three kinds of phosphonate ionic liquids were synthesized and used as lubricant additives in two hydrocarbon oils (mineral oil (MO) and poly alpha olefins (PAO)) for steel/steel contact. The tribological performance of the lubricants was evaluated on an Optimal SRV-IV oscillating friction and wear tester. The worn steel surfaces were analyzed by a JSM-5600LV scanning electron microscope and a PHI-5702 multifunctional X-ray photoelectron spectrometer. Results show that phosphonate ionic liquids improved the tribological performance of the hydrocarbon oils effectively. The mechanism was that the phosphonate ionic liquids adsorbed onto the steel surface because of its high polar to form the adsorption films. Furthermore, the tribochemical reactions between the phosphonate ionic liquids and the steel surface occurred to form the tribochemical reaction films.