We report an adjustable magnetic resonance frequency from 1.45 to 2.54 GHz for NiFe2O4 nanoparticles which were prepared by a sol-gel process. X-ray diffraction and scanning electron microscopy results indicate that the samples are polycrystalline nanoparticles, and the size of the particles increases obviously with the thermal treatment temperature. The consequence of the surface composition suggests that the oxygen defects are present in the nanoparticle surface, and this surface magnetic state can show a strong surface anisotropy. With decreasing size of the particle, the surface magnetic effect is predominant, resulting in an increase of resonance frequency for NiFe2O4 nanoparticles. This finding provides a new route for NiFe2O4 materials that can be used in the gigahertz range.