In this study, the measurement of droplet size distribution by means of a laser diffraction technique in ultrasonic spray pyrolysis was investigated. Effects of precursor temperature, spray volume rate, carrier gas flow rate, and physicochemical properties of precursors were analyzed in detail. The results showed that the mean size of precursor droplets decreased with increasing precursor temperature, concentration, and addition of alcohol, while it increased with increasing spray volume rate and carrier gas flow rate. Bimodal droplet size distribution was observed probably due to droplet coagulation effect. A cyclone or an impactor was used to control the droplet size distribution. Uniform droplets were obtained using a cyclone that removed large droplets. For comparison, spray pyrolysis of salt aqueous solutions, such as ZrO(NO 3) 2 and Ni(NO 3) 2 , were also conducted. Particle size distribution of the spray pyrolyzed powders was analyzed using scanning electron microscopy. The results showed that the particle size distribution of prepared solid particles was strongly dependent on, and could be controlled by, variation in the corresponding properties of the sprayed droplets.