ZnO films are hydrothermally grown on ZnO-buffered c-plane sapphire substrates at a low temperature of 70 degrees C. A radio-frequency (RF) reactive magnetron sputtering has been used to grow the ZnO buffer layers. X-ray diffraction, scanning electron microscopy, and room temperature photoluminescence are carried out to characterize the structure, morphology and optical property of the films. It is found that the films are stress-free. The epitaxial relationship between the ZnO film and the c-plane sapphire substrate is found to be ZnO (0 0 0 1)parallel to Al2O3 (0 0 0 1) in the surface normal and ZnO [1 0 (1) over bar 0]parallel to Al2O3 [1 1 (2) over bar 0] in plane. Sapphire treatment, as such acid etching, nitridation, and oxidation are found to influence the nucleation of the film growth, and the buffer layers determine the crystalline quality of the ZnO films. The maximum PL quantum efficiency of ZnO films grown with hydrothermal method is found to be about 80% of single-crystal ZnO. (C) 2010 Elsevier B.V. All rights reserved.