Three phosphonate ionic liquids (1-butyl-3-methylimidazolium O-butyl phosphonate, N,N-dibutylammonium O-butyl phosphonate and N, N-dibutylammonium O-ethyl phosphonate) are used as additives in PEG for steel/steel contact. Moreover, the tribological performances and lubrication mechanism were studied. Results show that these ionic liquids can improve the tribological performances of the base oil significantly. The main factor that influences the tribological performances of the phosphonate ionic liquids/PEG blends is the competitive adsorption between the base oils and the additives. In addition, chemical reactions between the ionic liquids and the metal substrate are responsible for the excellent tribological performances of the phosphonate ionic liquids/PEG blends.