Abstract: Oleic acid (OA) surface modified TiO 2 nanoparticles with average diameter of 30 nm were chemically synthesized. The tribological properties of GCr15 (SAE52100) steel under the lubrication of OA TiO 2 nanoparticle/water were evaluated with a four ball tester. The rubbed surface of the steel was analyzed with X ray photoelectron spectroscopy (XPS) and electron probe microanalysis (EPMA). The results show that OA TiO 2 nanoparticle as additive in water exhibits good performance in antiwear and friction reduction as well as in load carrying capacity. The maximum non seizuer load ( p B value) can be raised by 6 10 times when 0.1 1.0 % OA TiO 2 nanoparticle was added into water. The results of surface analysis indicate that a boundary film mainly composed of TiO 2 and titanates formed on the rubbed surface. This contributes to decrease the friction and wear of GCr15/GCr15 steel pair.