Abstract: In absolute ethanol , ultrafine SiO₂ particles were prepared by hydrolyzing tetraethylorthosilicate using ammonia as catalyst and calcined at 500 ℃ for 1 h CeO₂ SiO₂ composite nanoparticle was obtained by immersing SiO₂ in sols with different cerium ion concentrations , which were prepared using cerium nitrate , diacetone , normal propyl alcohol as raw materials The phase composition , morphology , particle size and agglomerate of the samples were analyzed by X-ray diiffraction ( XRD ) , scanning electron microscope ( SEM ) , transmission electron microscope ( TEM ) and infrared spectroscopy ( FT-IR ) The slurry collocated by as-prepared CeO₂@SiO₂ composite nanoparticles was used to polish GaAs wafer ( 100 ) The polishing behavior of CeO₂@ SiO₂composite abrasive was characterized by Atomic Force Microscope ( AFM ) The results indicated that monodisperse , spherical CeO₂-coated SiO₂particles were prepared successfully The particle size was 400 -450 nm and SiO₂particle was uniformly coated by CeO₂. The amount of coated CeO₂ increased gradually with the increase of cerium ion concentration in the sol. After chemical -mechanical polishing by CeO₂@SiO₂ composite abrasive , ultra-smooth surface of GaAs ( 100 ) with a surface roughness ( R_a ) of O.819nm within 1μm×1μm area was obtained