Abstract: Preparation method of water-based nano hydraulic fluid was presented. Utilizing four-ball friction and anti-wear testing machines,an experimental study on wear-resistance and anti-friction performances of water-based nano hydraulic fluid with different nanoparticles concentrations and sizes were carried out. The results show that silica nanoparticles can improve the wear-resistance and anti-friction performances of the water-based hydraulic fluid. For 30 nm silica nanoparticles,friction coefficient,wearing scar diameter,temperature increment,and wearing amounts were the lowest at the concentration of 2.4%. For nanoparticles of different sizes,the wear increased firstly and then decreased by increasing the concentration of nanoparticles. The larger the size of nanoparticles,the lower the mass concentration for the best performances.