Cu-doped ZnO nanoparticles were prepared by a sol-gel method for the first time. XRD, XPS, UV-vis and FS techniques were used to characterize the Cu-doped ZnO samples. The photocatalytic activity was tested for methyl orange degradation under UV irradiation. The results show that the crystal sizes of ZnO and 0.5% Cu/ZnO nanoparticles with wurtzite phase are 32.0 and 28.5 nm, indicating that Cu-doping hinder the growth of crystal grains. The doped Cu element existed as Cu(2+). The optimal Cu doping concentration in ZnO is 0.5%. The optimal calcination condition is at 350 degrees C for 3 h. The MO degradation rate of 0.5% Cu/ZnO reaches 88.0% when initial concentration of MO is 20 mg/L, exceeding that of undoped ZnO. The enhanced charge carrier separation and increased surface hydroxyl groups due to Cu-doping contributed to the enhanced photocatalytic activity of 0.5% Cu/ZnO. (C) 2011 Elsevier B.V. All rights reserved.