We studied the geometrical structures and electronic properties of Si-doped TiO2 using spin-polarized density functional theory calculations. Our results show that the valence band maximum has a little raise in substitutional Si to O-doped TiO2, and the Fermi level is pinned in the conduction band edge, indicating typical n-type characteristic. In substitutional Si to Ti-doped TiO2, the electron transition energy from the valence band to the conduction band has a decrease about 0.25 and 0.2 eV in anatase and rutile TiO2, respectively, which may be responsible for the experimental redshift of optical absorption edge. (C) 2008 Elsevier B.V. All rights reserved.