Room temperature ferromagnetism (RTFM) was observed in both La-doped and pure ZnO nanoparticles synthesized by the sol-gel method. RTFM is intrinsic according to the results of X-ray diffraction and Xray photoelectron spectroscopy. The saturation magnetization (M-S), the remnant magnetization at zero field and coercive field are 5 x 10(-3), 7 x 10-4 emu g(-1), 100 Oe for Zn0.99La0.01O nanoparticles and 1.5 x 10 (-4), I X 10(-5) emu g(-1), 50 Oe for pure ZnO nanoparticles, respectively. The magnetization is enhanced greatly by doping of La. Furthermore, the M-S of Zn0.99La0.01O nanoparticles decreases from 0.005 to 0.001 emu g(-1) as the annealing temperature increases from 500 to 700 degrees C. The doping of La introduces more oxygen vacancies into ZnO. The decrease of annealing temperature also produces more oxygen vacancies in La-doped ZnO. These results indicate that the origin of the RTFM is related to oxygen vacancies. (C) 2014 Elsevier B.V. All rights reserved.