Nickel oxide particles were prepared by spray pyrolysis of aqueous solution of NiCl2·6H2O. In the reactor the salt droplets were first converted to hollow particles by drying and then they were collapsed by oxidation to reduce their size. Each oxide particle was composed of many small nuclei with voids among them due to extremely low rate of sintering. The particle size decreased with the temperature as the sintering and crystallization proceeded. The size as well as the crystallinity of the particles increased with the initial salt concentration. When the salt droplets were preliminarily dried in diffusion dryer before entering the reactor, the collapse of the particles was considerably reduced, resulting in lower hollowness and higher sphericity. Numerical simulation on the drying of the droplets provided insight on the initial stage of spray pyrolysis.