ZnO nanoparticles were synthesized by drop-wise addition of ZnCl2 solution to KOH, in different temperatures and sonication output powers. The morphologies and structure of resulting materials were characterized by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The morphology of the products was determined to be a mixture of nano rods as well as nanoparticles depending to sonication output power and temperature. A reduction in nanoparticles and aggregates sizes as well as amount of byproducts was observed by increasing the sonication output power. A combination of sonication output power and temperature was also studied on the properties of the product. At 70 degrees C and 45 W sonication output power, the smallest nanoparticles as well as narrower range of sizes in addition to minimum amount of byproducts was obtained in this work. (C) 2007 Elsevier Ltd. All rights reserved.