Abstract: We have studied the electrical properties of rutile TiO_2-xN_x compound which x is the fraction of Ni impurities and was chosen to be 0.0625, 0.125, 0.25 and 0.5. In the calculations, we have adopted a pseudopotential and supercell approach based on the Density Functional Theory (DFT) with GGA approximation for the Exchange-correlation potential as implemented in SIESTA code. We have calculated the band structure and Density of State (DOS) and therefore, the changes of the energy gap of this compound with nitrogen substitution in oxygen sites. The quality of the used pseudopotential was tested by comparing the calculated band structure and DOS using Full Potential Linear Augmented Plane Wave (FP-LAPW) and pseudopotential methods. The result shows that the energy gap of TiO_2-xN_x has decreases considerably due to the appearing of the N-2p orbitals in the bottom of the energy gap which is in good agreement with the recent experimental researches about the red shift of the threshold absorbance frequency of this compound which has made it an interesting photocatalysis in the visible spectrum of the solar radiation.