Abstract: Two kinds of guanidinium ionic liquids were synthesized. The thermal stabilities of the ionic liquids were evaluated by a hermogravimetric analyzer. Their tribological properties were investigated and compared with alkyl imidazole ionic liquids and liquid paraffin by SRV friction-wear test. The results show that guanidinium ionic liquids were the best lubricant among the tested lubricants for steel/Al contacts. The images of worn surface were observed using scanning electron microscope. The X-ray photoelectron spectrometer analysis was carried out to explore the lubrication mechanism. The results show that complex tribochemical reactions occurred and the boundary lubrication films were formed in aluminum/steel friction pair interface during the sliding process.