In this paper, we report the preparation and characterization of both the ZnO nanocrystalline thin films and nanopowder by ultrasonic spray pyrolysis technique. ZnO films were grown on polished Si(100) and amorphous glass substrates at different deposition temperature range varying from 200 to 500 degrees C. Both orientation and the size of the crystallites were found to depend on the substrate and substrate temperature. XRD patterns of the films deposited on glass show strong c-axis oriented crystal structure with preferred (002) orientation, while those deposited on Si show predominant (100) orientation. Field emission scanning electron microscopy (FESEM) further confirms the a-and b-axis orientation in ZnO films on Si substrate with hexagonal columnar like grains and c-axis orientation in case of glass substrate with grains perpendicular to the substrate. The morphology and crystallite size of the ZnO nanopowder characterized by FESEM and TEM revealed that the powder consisted of the mixture of nanoparticles with particle size of 50-100 nm. The XRD results indicated that the synthesized ZnO powder had the pure wurtzite structure with lattice parameters a and c of 3.244 and 5.297 nm, respectively. High-temperature XRD studies of ZnO nanopowder showed that the crystallite size increased with increasing temperature with a systematic shift in peak positions towards lower 20 values due to change in lattice parameters. Temperature dependence of the lattice constants shows linear increase in their values. Diffraction patterns of ZnO nanopowder obtained from TEM were also in agreement with the XRD results. The synthesized powder exhibited the estimated direct band gap (E,) of 3.43 eV. (c) 2007 Elsevier B.V. All rights reserved.