Abstract: Ceria nanoparticles were prepared by precipitation method.The structural feature and morphology were characterized by XRD and TEM.The effect of calcination temperature on the morphology and 500SN base oil tribological properties of the CeO2 nanoparticles was investigated.The results show that the ceria nanoparticles calcined at temperature below 400 ℃ could reduce the friction coefficient and improve the tribological properties of 500SN base oil.However,the ceria nanoparticles calcined at temperature above 600 ℃ could increase the friction coefficient and the wear scar diameter.The higher the calcination temperature,the more integral the crystal structure of the CeO2 nanoparticles.The particles with sharp corners transformed from nearly spherical into polyhedral,which led to the decrease of the tribological properties.When the calcining temperature was lower,the existence of the amorphous composition on the surface of crystal particles could improve the tribological properties of base oil.