Fine particles of calcium oxide doped cerium dioxide, 2-4 nm in diameter, were prepared by the chemical reaction of CeCl3-CaCl2 mixed aqueous solution and NaOH aqueous solution at pH 6-12 and 40degreesC followed by the oxidation with hydrogen peroxide. Doping of CaO with CeO2 resulted in decreasing the particle size and consequently, increasing UV shielding efficiency and transparency to visible light. The particle shape of ceria changed significantly depending on the reaction condition, i.e., rod-like particles and spherical particles were formed by the H2O2 oxidation of cerium trihydroxide pH above 8 and below 7, respectively.