Diluted magnetic oxide of Ce 0.97 Co 0.03 O 2-d powder was synthesized by standard solid-state reaction method. Oxidized or reduced samples were obtained via O 2 or Ar/H 2 annealing, respectively. Magnetic investigations found that the as sintered Ce 0.97 Co 0.03 O 2-d is ferromagnetic at room temperature. The saturation magnetic moments (M S ) are highly enhanced by Ar/H 2 annealing, while sharply diminished by O 2 treatments. X-ray photoelectron spectroscopy (XPS) measurements show that Ce 4 + ions are partially reduced to Ce 3 + with additional oxygen vacancies (V O ) being introduced by Ar/H 2 annealing. The increase of V O by Ar/H 2 annealing is further proved by Raman spectra in which a new Raman scattering peak appearing at the frequency of 465 cm À 1 . This fact contributes to the enhanced ferromagnetism observed in reduced Ce 0.97 Co 0.03 O 2-d . On the other hand, V O density is reduced via O 2 annealing and thus leading to a decreased M S . On all accounts M S is found to be in direct ratiodependent on V O density in Ce 0.97 Co 0.03 O 2-d . The experimental results are well consistent with the F-center mediated ferromagnetism mechanism recently found in some transitional metal (TM) doped oxides, and thus is very important in understanding the mechanism of the ferromagnetism (FM) origin in many TMs-doped insulating oxides.