The most suitable method to improve the photocatalytic activity of semiconductors is doping of noble metallic nanoparticles on them. In this research, Au, Ag and Pd noble metals were separately doped on ZnO nano photocatalysts by borohydride reduction method. The as-prepared doped on ZnO nano photocatalysts were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy, (FESEM), energy dispersive spectroscopy (EDS), inductively coupled plasma optical emission spectroscopy (ICP-OES), ferromagnetic resonance (FMR) and UV-Vis diffuse reflectance/absorbance spectroscopy (DRS). The photocatalytic activities of nano photocatalysts were evaluated by the degradation of Congo red (CR) dye under UV irradiation. It was found that noble metals doped ZnO nano photocatalysts could significantly increase the photocatalytic activity of ZnO. When we compared the enhancing effects of Au, Ag and Pd, it was found that Pd was more effective than others. The differences in activities of various noble metals doped ZnO may be related differences in the amount of reducing noble metals, in their work functions and in their heights of Schottky barriers. The results of these studies demonstrate that doping of noble metals on ZnO can delay significantly the recombination process of the electron hole pairs generated by the photon absorption and consequently improve the photo catalytic activity of the ZnO. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.