Abstract: A novel oil-soluble rare earth compound (La-Ci8) free of sulfur and phosphorus was synthesized as lubricant additive. The thermogravimetric analysis (TGA) shows that it possesses good thermal stability. The tribological properties of La-Ci8 added to liquid paraffin were evaluated using a four-ball tester. The results show that it exhibits the better anti-wear properties. When combining with a certain amount of organic ligands, it can further improve the friction-reducing and anti-wear properties. The characterizations of the worn surface were analyzed by X-ray photoelectron spectroscope (XPS) and Auger electron spectroscope (AES). The result indicates that La-Ci8 was adsorbed on rubbed surface, some of which reacted with iron from the steel ball to form La by a redox reaction during friction process. La element deeply penetrated into sublayer of surface. Both the adsorption film containing La(III)carboxylate and the reaction film containing La and Fe(II)carboxylate contribute to the formation of a high performance composite boundary lubrication film.