Tribological properties of five anti-wear additives for a steel-copper contact were investigated. It was found that the tribological performances are closely related to the molecular structure of additives. The protic ionic liquid anti-wear additive AW316 exhibits the best tribological performance with the lowest mean friction coefficient of 0.082, and the smallest wear volume, which is more than one order of magnitude smaller than base oil. Transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS) tests reveal that a 10-15 nm thickness uniform boundary lubrication film composed of oxides, phosphates, and cuprous oxide was formed on the copper disc, which was responsible for its outstanding tribological performances.