The purpose was to elucidate the production process of cerium dioxide particles, and the influence of the production conditions on the pyrolysis process of cerium nitrate droplet and the powder characteristics of the produced particles were examined. Primary, a single droplet of cerium nitrate solution was suspended in the hot air to examine the pyrolysis process of cerium nitrate solution to cerium dioxide particle. As a result of the observation, the pyrolysis process of cerium nitrate solution consisted of "foaming" and "solid-phase formation", which were characteristic phenomena and the process was categorized into four phases; evaporation, foaming, solid-phase formation and pyrolysis. Considering this result, cerium dioxide particles were produced by spraypyrolysis. Particularly, initial droplet diameter, initial metal concentration and air temperature were attracted as the production conditions and the influence of them on the size distribution, specific surface area and shape of the produced particles were examined. The mono-dispersed spray with narrow droplet size distribution was injected into a hot air stream and cerium dioxide particles were collected. The size distribution of produced particles was wide in comparison with the initial droplet size distribution, because the sample droplets were scattered or broken up by foaming in the process. The scattering or breakup of droplets due to foaming was promoted with increase of air temperature, the frequency of fine particles increased and mode diameter of produced particles decreased. From the observation with SEM and TEM, the produced particles were porous.