The Co-doped ZnO thin films were prepared on c-cut sapphire substrates by magnetron cosputtering, and then annealed at various temperatures in vacuum. Magnetic measurements indicate that all the films are ferromagnetic at room temperature and the magnetization of the annealed Zn0.88Co0.12O films is increased about one order of magnitude in comparison with the corresponding as-deposited one. The enhancement of magnetization is possibly due to the fact that the generation of oxygen vacancies during annealing in vacuum increases the carrier (electron) concentration, which is consistent with the mechanism of carrier-induced ferromagnetism. Optical spectrometry indicates that Co2+ enters the tetrahedral sites of the wurtzite structure of ZnO host and substitutes for Zn2+.