Monocrystalline ZnO thin films on p-GaN/sapphire (0001) substrate were grown by single step hydrothermal technique at 90 degrees C. ZnO thin films were grown in aqueous solution of zinc nitrate and ammonium hydroxide. The X-ray diffraction, scanning electron microscopy and room temperature photoluminescence analysis were carried out to characterize structural, morphological and optical properties of the films. The thin films revealed single crystalline nature and wurtzite symmetry. The films were c-axis oriented and have honeycomb like pitted surface morphology. The epitaxial relationship between ZnO film and p-GaN buffer layer was observed to be (0001)[11 (2) over bar0](ZnO)parallel to(0001)[11 (2) over bar0](GaN). Sharp luminescent peak centered at 376 nm due excitonic emission and broad deep level emission peak were obtained from room temperature photoluminescence measurement of the ZnO thin film. (C) 2008 Elsevier B.V All rights reserved.