Nanocrystalline ZnO thin films were prepared using modified liquid flow deposition method (LF method). The effects of annealing in temperature range of 350-550 degrees C on the structural and optical properties of the ZnO films have been studied. The structure, surface morphology, chemical composition and optical properties of the films were determined using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis using X-rays (EDAX), UV-vis-NIR and photoluminescence (PL) spectroscopy. X-ray diffraction measurements showed that the annealed ZnO films were polycrystalline with (1 00), (0 0 2) and (1 0 1) oriented crystallites of hexagonal wurtzite structure and full width half maximum (FWHM) of (0 0 2) XRD peak is observed to decrease as annealing temperature increases above 350 degrees C. The film annealed at 550 degrees C possesses c-axis oriented hexagonal wurtzite ZnO nanoparticles. The crystallinity as well as grain size of the films was found to increase on post-annealing. SEM micrographs of the films reveal the presence of hexagonal shaped nanoparticles and EDAX measurements confirm the removal of impurities after annealing treatment. The optical band gap of ZnO films initially blue shifted (3.39-3.41 eV) when annealed at 350 degrees C and a red shift (3.41-3.32 eV) was observed above the annealing temperature 350-550 degrees C. Room temperature photoluminescence (PL) measurement of the films showed UV and defect related deep level emissions including blue, green and yellow emissions in the visible region. The possible mechanisms of these emissions were studied in detail. (C) 2010 Elsevier B.V. All rights reserved.