Pure ZnO and Ni-doped ZnO rods have been prepared by hydrothermal method at a temperature of 120 degrees C. The morphological, structural, magnetic and optical properties of the as-prepared rods were investigated by means of field emission scanning electron microscopy, X-ray diffraction, vibrating sample magnetometer and photoluminescence. All the samples are radial-grown hexagon rods with diameter from 470 to 720 nm and length of 4-6 mu m. X-ray diffraction shows that the rods have single crystalline wurtzite structure without other impurity phases. The pure ZnO rods and Ni-doped ZnO rods have ferromagnetism at room temperature, and the special saturation magnetization deduces with the increasing diameter of rods. These results reveal that the saturation magnetization of the ZnO rods depends on the surface-to-volume ratio of rods rather than the Ni doping concentrations. The photoluminescence spectra studies show the same diameter dependences of oxygen vacancies as that of magnetization, which demonstrates that oxygen vacancies at surface of rods play an important role in introducing ferromagnetism. The annealing in rich oxygen and reducing atmospheres confirms this argument further. (C) 2010 Elsevier B. V. All rights reserved.