Nanosize ceria-zirconia ultrafine particles were prepared by the microemulsion method. HREM micrographs revealed that nanometer-sized particles were prepared and the shapes of them were uniform. The as-prepared particles were amorphous and the specific surface area of them was very large. The particle size ranged between 2 and 11 nm, and the mean particle size was evaluated to be 4 nm. The catalytic activity of the ceria-zirconia ultrafine particles impregnated on γ-Al 2 O 3 powders for CO oxidation was compared with that of the conventional catalyst prepared by the coprecipitation method. Even after the heat treatment at 1273 K for 5 h, the catalysts prepared by the microemulsion method still showed a higher activity than conventional ones. This finding suggests that the preparation method presented in this study is very useful to synthesize the catalysts of high activities and high heat resistance.