We present the strain evolution of ZnO/sapphire(0001) thin films studied by X-ray diffraction, scanning electron microscopy, and atomic force microscopy. The effect of input radio frequency (rf) sputtering power and growth temperature was examined. Initially the highly strained flat ZnO layers were grown. At samples deposited at higher temperatures than 500 degrees C. the strain was fully relaxed by nucleating the ZnO nanograins as the film thickness increased. However, we found that the strain was partially relaxed at the samples grown at lower temperatures, which was attributed to the insufficient surface undulations. For the effect of rf input power, a consistent behavior of strain relaxation independent of the rf input power was observed. The ZnO nano-grains were enlarged and transformed to well-defined hexagonal nano-discs with film thickness. (C) 2010 Elsevier B.V. All rights reserved.