In situ formed ionic liquids (ILs) as high temperature lubricants can be obtained by mixing polyol esters (pentaerythritol oleate or trihydroxymethylpropyl trioleate) with lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in molar ratios of 1/0.5, 1/1 and 1/1.5. Thermal and rheological analysis indicate that the in situ formed ILs, [Li(polyol ester)]TFSI, with various molar ratios are more resistant to high temperatures and substantially stronger than the polyol ester base oils. Tribological results revealed that these in situ forming ILs with molar ratios of 1/1 and 1/1.5 possessed excellent friction reduction and antiwear performance for steel/steel contact at 300 °C. Moreover, their tribological properties were significantly better than the pure polyol ester base oils and perfluoroalkyl polyether (PFPE, as a reference lubricant) under the same conditions. The results of SEM-EDS show that tribo-films formed on the worn steel discs surface lubricated by [Li(polyol ester)]TFSI. The XPS analytical results further proved the formation of a boundary lubrication films which composed of Fe₂O₃, Fe₃O₄, Fe(OH)O, FeF₂, FeSO₄ or Fe₂(SO₄)₃ and C–O bonding. This surface-protective film is believed to be responsible for the excellent tribological performances of in situ forming ILs using at elevated temperature.