Abstract: Nanometer CeO2 particles were prepared by ultrasonic atomization process using cerous nitrate (Ce(NO3)3·6H2O) and ammonium acid carbonate (NH4HCO3) as starting materials.The effect of atomization temperature,calcination temperature,reactants concentration and surface active agent on the phase compositions of precursors and the size and morphology of CeO2 particles were studied by the techniques of XRD and TEM.The results show that the phase compositions of the precursors related to the atomization temperature,and CeO2 nanoparticles prepared at 60 ℃ had less agglomeration and more uniform morphology than other temperature.Increasing the calcination temperature would result in the rapid growth of CeO2 particles and decreased the specific surface area sharply,so the better calcination temperature was 200 ℃.Decreasing reactants concentration had little effect on the particle size,but could improve the agglomeration state obviously,so the selected solution concentration was 0.2 mol·L-1.Surface active agent are favorable for decrease the surface tension of the reaction solution,and smaller and more homogeneity spherical CeO2 particles with particle size of only 3 nm were obtained with 10% ethanol and 0.5% polyethyleneglycol(PEG) as surface active agent under aboving optimum conditions.