In this paper, by using density functional theory, Mg and Nb-Mg co-doping of TiO2 anatase (101) surfaces are studied. By studying the formation energy of the defects and the bond length distribution of the surface, it is shown that Mg defects tend to stay as far as possible to induce least possible lattice distortion while Nb and Mg defects stay close to each other to cause less stress to the surface. By investigating band structure of the surface and changes stemmed from the defects, potential effects of Mg and Mg-Nb co doping of TiO2 surface on dye-sensitized solar cells are investigated. In this study, it is shown that the Nb-Mg co-doping could increase Jsc of the surface while slightly decreasing V-oc compared to Mg doped surface, which might result in an increase in efficiency of the DSSCs compared to Nb or Mg doped surfaces. (C) 2016 Elsevier B.V. All rights reserved.