Spinel type Mg-doped lithium titanium oxides of Li(4-2x)MgxTi(5-x)/O-3(4) has been investigated in terms of the relationship between the crystal structure and the ionic conductivity by both computational and experimental techniques. Reduction of the energy barrier for Li+ hopping was indicated by the computations based on the first-principles density functional theory (DFT) and long-range Coulombic interactions by doping Mg2+ into tetrahedral sites of spinels, including increase of ionic conductivity of Lit However, the experimental measurements for activation energies and ionic conductivities of Li were opposite to the expectation from computational study. One of the reasons for the discrepancy was neglecting defect formation energy in the spinel Li4/3Ti5/3O4 in the computations, which was supported by computations assuming Schottky-like Li vacancy formation. In addition, severe increase of activation energy was indicated by doping Mg2+ ions at the composition range, x>0.4 in experiments. The anomalous increase of activation energy was discussed by adopting percolation theory. (C) 2010 Elsevier B.V. All rights reserved.