Global warming has generated great concern worldwide. One way to control this problem is to use clean renewable energy new sources. Among the energy sources, we can mention hydrogen gas, produced by water photocatalysis by mean of a semiconductor material. In this work, we report a study about band gap and absorption spectra by mean of the density functional theory of the anatase allotropic form of titanium dioxide doped with Ruthenium. The results indicated systematic decrease of the band gap and increase of the absorbance at the visible region with the increase of the amount of dopant.