A material removal mechanism of ceria particles with different sizes in a glass polishing process was investigated in detail. Contrast polishing experiments were carried out using ceria slurries with two kinds of particle sizes and different amounts of hydrogen peroxide (H 2 O 2 ) added in the slurries. The Ce 3+ ions on the surface of the ceria particles were gradually oxidized to Ce 4+[/sub] with increased H 2 O 2 concentration. It was found that the material removal rate (MRR) decreased sharply with an increasing concentration of H2 O 2 . There was no material removal when the concentration reached 2.0% for nanoparticle slurry. Nevertheless, the application of microparticles made the MRR decrease to a constant value when excessive H 2 O 2[/sub] was added. By comparison, we conclude that the material is removed by chemical reaction for ceria nanoparticles, while chemical reaction and mechanical abrasion simultaneously take place for ceria particles with sizes at scale of micrometers in the glass polishing process. It is clearly demonstrated from the experimental results that Ce 3+ instead of Ce 4+ ions play an important role in chemically reacting with the glass surface. An ultrasmooth surface with root-square-mean roughness of 0.272 nm was obtained after being polished by ceria nanoparticles.