The nano-grade cerium oxide particles were formed by high temperature precipitation. Two kinds of seeds, one was 3 nm (Sample A) in diameter and the other was 8 nm (Sample B) in diameter, were prepared by the different methods. The urea solution as the precipitant was titrated to the solution of ammonium cerium nitrate with the seeds. The morphology of the final particles was affected by the kind of seeds. The particles aggregated to form the agglomerate of about 20 nm in diameter as the Sample A was a seed. If the Sample B was a seed, the product was spherical and pseudo single crystal with 40-50 nm in diameter. Both final products would change to the agglomerate of 8 nm primary particle after further aging.