The Cr-doped zinc oxide (Zn0.97Cr0.03O) nanoparticles were successfully synthesized by sol-gel method. The relationship between the annealing temperature (400 degrees C, 450 degrees C, 500 degrees C and 600 degrees C) and the structure, magnetic properties and the optical characteristics of the produced samples was studied. The results indicate that Cr (Cr3+) ions at least partially substitute Zn (Zn2+) ions successfully. Energy dispersive spectroscopy (EDS) measurement showed the existence of Cr ion in the Cr-doped ZnO. The samples sintered in air under the temperature of 450 degrees C had single wurtzite ZnO structure with prominent ferromagnetism at room temperature, while in samples sintered in air at 500 degrees C, a second phase-ZnCr2O4 was observed and the samples were not saturated in the field of 10000 Oe. This indicated that they were mixtures of ferromagnetic materials and paramagnetic materials. Compared with the results of the photoluminescence (PL) spectra, it was reasonably concluded that the ferromagnetism observed in the studied samples was originated from the doping of Cr in the lattice of ZnO crystallites. (C) 2009 Elsevier B. V. All rights reserved.