TiO2 thin films were prepared by direct current magnetron sputtering on glass substrates, then were implanted by cobalt ions, and finally annealed at 500 ℃ for 50 min. Specimens for transmission electron microscopy were prepared by peeling-scattering technology, and were observed in situ by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and high resolution transmission electron microscopy (HRTEM). The films were identified as anatase structure. The Co ions exist mainly in a thin layer beneath the surface of the films. The implanted Co ions damage the TiO2 crystals and some of them exist as CoO. The annealing treatment could repair the TiO2 grains and make the Co ions diffuse into the TiO2 lattice. The vibrating sample magnetometer (VSM) measurements show that the Co ion implanted TiO2 films are ferromagnetic at room temperature and the annealing treatment in vacuum could enhance the ferromagnetism, which could be explained within the scope of the BMP theory.