N-doped Cu2O films are deposited at different temperatures by sputtering a CuO target in the mixture of Ar and N2. By the analysis of transmission spectra, it is found that the N-doped Cu2O films are changed into a direct allowed band-gap semiconductor and the optical band gap energy is enlarged to 2.52±0.03 eV for the films deposited at different temperatures. The first-principles calculations indicate that the energy band gap increase by 25%, which is in good agreement with the experimental result. The change from a direct forbidden band-gap transition to a direct allowed band-gap transition can be attributed to the occupation of 2p electrons of N at the top of valence band in the N-doped Cu2O film.