Ashless organic friction modifier can reduce the friction coefficient of hydrodynamic lubrication,which provide new idea to improve the hydrodynamic lubrication performance.The monoolein glycerol,oleic acid,pentaerythritol tetraisostearate and Span 80 were used as ashless friction modifier.Four kinds of lubricant samples were prepared by mixing 5% mass fraction of ashless friction modifier with base oil PAO 5,and their hydrodynamic lubrication performances were studied by fluid dynamic pressure lubrication test rig.Results show that the ashless friction modifiers have certain friction reduction effects under boundary lubrication and mixed lubrication,while the friction reduction effect is more significant under hydrodynamic lubrication.The lubricant containing 5% mass fraction of pentaerythritol tetraisostearate shows the best friction reduction performance,which reduces the friction coefficient of hydrodynamic lubrication by 19.6% compared with base oil.Molecular dynamic simulation results show that the ashless friction modifier produces boundary slip phenomenon on friction surface,reduces the intertwine of carbon chains between molecules,and thus reduces the friction coefficient under hydrodynamic lubrication.