Single-crystalline ZnO nanorod arrays with diameters of 60-120 nm and lengths of 2 mu m have been grown epitaxially on GaN (0 0 1) by hydrothermal synthesis at 100 degrees C. The ZnO nanorods exhibit true epitaxial growth, in contrast to most of the previously reported methods, which involved the nucleation on a ZnO buffer layer pre-deposited on the substrate. An unprecedented high degree of c-axis orientation is achieved in this hydrothermal epitaxial growth. The effect of growth conditions on the deposition of ZnO nanorods was systematically studied. We showed that by changing the molar ratio of the reactants, we can control the growth morphology of the ZnO nanorod arrays. The optimal conditions to produce high-density single-crystalline ZnO nanorods with c-axis orientation have been identified. The mechanism affecting ZnO nanorod epitaxial nucleation and growth on GaN is discussed. The ZnO nanorods exhibited ultraviolet photoluminescence (PL) at 388 nm and the PL shows a decay lifetime of 58 ps. (c) 2006 Elsevier B.V. All rights reserved.