Abstract: The tribological properties of surface modified Fe3O4 and AlOOH nanoparticles as additives in liquid paraffin were evaluated and compared by a four-ball tester. The results show that the friction-reduction and antiwear properties and the PB value of liquid paraffin are improved by adding AlOOH or Fe3O4 nanoparticles as additives. The AlOOH nanoparticles exhibit better friction reduction, antiwear properties due to the lamella structure. The optimum concentration of nanoparticles shows the tendency of increase as the diameter of nanoparticles increases, and high volume concentration must be required to achieve the same PB value for nanoparticles with larger size. The results of the worn surface analysis with SEM, AFM and XPS show that the AlOOH or Fe3O4 nanoparticles can deposit on the worn surface and separate the direct contact between the asperities of the worn surface, which is effective in reducing the plough and adhesion between asperities.