Zinc oxide thin films were deposited on silicon substrates via hydrothermal method. Microstructures, surface topographies and optical properties of ZnO thin films were systematically investigated by X-ray diffraction, atomic force microscopy and fluorescence spectrophotometer. The mean grain size and surface roughness of the thin films decrease first and then increase with increasing the concentration of zinc nitrate hexahydrate. The photoluminescence spectra of ZnO thin films, excited by the 240, 320, 360, 380 and 400聽nm excitation wavelength, were investigated in detail. Based on our analysis, it can be noted that mechanisms of the ultraviolet, violet and blue emissions are attributed to the transitions from the localized levels below the conduction band, zinc vacancy, interstitial zinc and extended interstitial zinc levels to the valance band, respectively. Blue iolet emissions of ZnO have great potential in light emitting and biological fluorescence labeling applications.