During chemical mechanical polishing (CMP) for a dielectric layer, Ce³⁺, which is the active site on the surface of ceria, has a significant effect on the material removal rate (MRR). Ceria nanoparticles were synthesized using a hydrothermal method with different dopant concentrations of lanthanide elements (La and Nd) to increase the concentration of Ce³⁺. An oxygen vacancy was formed in the ceria doped with the trivalent lanthanide ions, which reduced the surrounding Ce⁴⁺ to Ce³⁺. The concentration of Ce³⁺ increased up to 32.57% when the amounts of dopants were increased and Nd was doped. The improved reactivities of the particles by doping were verified by measuring the reduction of the work function of the doped ceria nanoparticles and the increase in the adhesion force with SiO₂. The polishing performance of Nd-doped ceria at high concentrations achieved an MRR up to 3.62 times higher than those of the comparison groups.