An oil soluble rare earth compound, samarium isooctoxyborate, was synthesized and used as an antiwear and extreme pressure additive for lubricating oils. Its tribological performances were evaluated with a four ball machine and a ring on block rig. In addition, surface analyses with Auger electron spectroscopy and X ray photoelectron spectroscopy were also conducted to study the surface film formed by the additive under boundary lubrication conditions. The test results show that the additive exhibits good friction reducing ability, good antiwear property and excellent extreme pressure performance. Surface analyses further disclose the coexistence of organic film, oxidation film and permeating film on the rubbing surface, which contributed to improve the tribological properties of the base oil. An oil soluble rare earth compound, samarium isooctoxyborate, was synthesized and used as an antiwear and extreme pressure additive for lubricating oils. Its tribological performances were evaluated with a four ball machine and a ring on block rig. In addition, surface analyzes with Auger electron spectroscopy and X ray photoelectron spectroscopy were also conducted to study the surface film formed by the additive under boundary lubrication conditions. The test results show that the additive exhibits good friction reducing ability, good antiwear property and excellent extreme pressure performance. Surface analyzes further disclose the coexistence of organic film, oxidation film and permeating film on the rubbing surface, which contributed to improve the tribological properties of the base oil.