TiO2 films are treated using a magnetron nitrogen-plasma apparatus that can generate high density plasma to realize nitrogen-doping. The films are treated for 5 min with nitrogen plasma at a discharge power from 30 to 100 W by a radio frequency generator operating at 13.56 MHz. The plasma treated films show visible light absorption in the range of around 400-500 nm. light absorption above approximately 500 nm is also observed in the film plasma treated at a high discharge power. It is confirmed from N-1s and Ti-2p X-ray photoelectron spectra of the films that Ti-N bonds and oxygen vacancies are formed. Visible light activity of photocatalysis in the film increases with increasing discharge power and then decreases. Such an increase and decrease in the visible light activity are due to the formation of Ti-N bonds and oxygen vacancies, respectively. The visible light activity is improved with the optimum plasma treatment. (C) 2008 Elsevier B.V. All rights reserved.