Photodegradation of methyl orange (MO) under 365-nm irradiation has been performed to investigate the photocatalytic activities of the nc-Au/ ZnO nanorod composites in comparison with those of the ZnO nanorods and the ZnO film. The ZnO nanorods and the ZnO film were deposited on the Si substrates using chemical vapor deposition. The apparent first-order rate constant of MO photodegradation using the ZnO nanorods is two orders magnitude larger than that using the ZnO film. Photosyntheses of Au nanoparticles on the ZnO nanorods were conducted in HAuCl4/ethanol solutions under 365-nm irradiation. The diameters and the densities of the An nanoparticles formed on the surface of ZnO nanorods are tunable through varying the HAuCl4 concentration and the irradiation period. The enhancement of the photocatalytic activity for degradation of MO is achieved by loading An nanoparticles with sizes smaller than 15 nm on the ZnO nanorods and is more pronounced as the size of the Au nanoparticles is reduced to 5 nm. However, the photocatalytic activity of the nc-Au/ZnO nanorod composite is much lower than that of the Zn nanorods when the diameter of the nc-Au is enlarged to 30 nm. The photocatalytic activity of the nc-Au/ZnO nanorods for degradation of MO is dependent on the diameter and the density of the nc-Au. (c) 2006 Elsevier B.V. All rights reserved.