Heteroepitaxial ZnO films are successfully grown on nondoped GaN-buffered Al2O3 (0001) substrates in water at 90 degrees C using a two-step process. In the first step, a discontinuous ZnO thin film (ca, 200 nm is thickness) consisting of hexagonal ZnO crystallites is grown in a solution containing Zn(NO3)center dot 6H(2)O and NH4NO3 at c. pH 7.5 for 24 h. In the second step, a dense and continuous ZnO film (ca. 2.5 mu m) is grown on the first ZnO thin film fin a solution containing Zn(NO3)center dot 6H(2)O and sodium citrate at ca. pH 10.9 for 8 h. Scanning electron microscopy. X-ray diffraction, UV-vis absorption spectroscopy, photoluminescence spectroscopy, and Hall-effect measurement are used to investigate the structural, optical, and electrical properties of the ZnO films. X-ray diffraction analysis shows that ZnO is a monocrystalline wurtzite structure with an epitaxial orientation relationship of (0001)[11 (2) over bar0]ZnO parallel to(0001)[11 (2) over bar0]GaN. Optical transmission spectroscopy of the two-step grown ZnO film shows a bandgap energy of 3.26 eV at room temperature. A room temperature photoluminescence spectrum of the ZnO film reveals only a main peak at ca. 380 nm without any significant defect-related deep-level emissions. The electrical property of ZnO film showed n-type behavior with a carrier concentrations of 3.5 x 10(18) cm(-3) and a mobility of 10.3 cm(2) V-1 s(-1).