Abstract: This work focused on studying the tribological performance of IL additives in the mixed and hydradynamic lubrication regimes. Polyethylene glycol (PEG-400) was used as the base fluid, ILs were synthesized in situ by dissolving lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in PEG. Mini Traction Machine was used to measure the friction coefficients with entrainment speeds at room temperature, 60 ℃ and 80 ℃ and at various slide-roll ratios for confirming the effect of IL additives on the tribological properties, and the rheological behavior of PEG was also investigated. This work would provide a new research method for further study of the lubrication mechanism of ionic liquids, which is of great significance for guiding the design of new ionic liquid lubricating materials.