Abstract: Three phosphate ionic liquids (butylammonium dibutylphosphate,dibutylammonium dibutylphosphate, and tetrabutylammonium dibutylphosphate) were synthesized with the cation moiety of alkylammonium and the anion moiety of phosphate, and evaluated as lubricants for the contact of steel/tin bronze by using an optimol SRV oscillating friction and wear tester under high temperature. The test results indicate that phosphate ionic liquids yielded commendable friction-reducing/anti-wear properties, and were superior to the conventional ionic liquid lubricant 1-methy-3-hexylimidazolium hexafluorophosphate. High temperature friction tests demonstrate the synergy effect between phosphorus and nitrogen elements. Lubrication mechanism of phosphate ionic liquids was analyzed by using scanning electron microscopy and X-ray photoelectron spectroscopy. As can be seen from a comparison figure of the wear scar morphology and XPS spectra of the characteristic elements,a boundary lubricating film exhibiting the protective effect has been formed on the worn surface. Thus alkylammonium phosphate ionic liquids performed excellent tribological properties.