Abstract: Precursor cerium oxalate was prepared by mechanical ball milling solid state reaction with CeCl₃·7H₂O and H₂C₂O₄·2H₂O as raw materials at ambient temperature. Ultrafine CeO₂ powder was then obtained by the thermal decomposition of precursor.The precursor powder′s change during the decomposition process was characterized by TG-DSC.Composition，size，and morphology of the product were also studied by XRD and SEM.The optimal process conditions were obtained as follow: filling ratio of mechanical ball milling was 40%，mass ratio of balls to raw material was 15∶1，grinding time was 2.5 h，calcining time was 2 h，and calcining temperature was 400 ℃.Under those conditions uniform and well-dispersed CeO₂ with particle size of about 300 nm was prepared.

Key words: ultrafine CeO₂, mechanical ball milling, solid state reaction, process parameters, micro morphology