The effect of mechanical strain on the electrical properties of Gd-doped CeO2 (GDC) thin films, which was induced by the structural change in the crystalline lattice, was investigated through microstructural and electrical characterization of the GDC films. The electrical conductivity was measured on GDC films grown epitaxially in the < 111 > direction on the (0001) surface of sapphire, as a functions of the film thickness in the range 52-403 nm and temperature in the range 500-700 degrees C. It was found that the activation energy decreases from 1.06 to 0.69 eV with increasing thickness. It is attributed to the in-plane compressive stress which originated from the lattice mismatch at the film-substrate interface. From the atomic scale simulation with ab-initio method we found that the activation energy for oxide ion migration was closely correlated with the distortion of crystalline lattice. Here we also suggested proper oxygen migration model under anisotropic lattice distortion which can explain the variation of activation energy for the oxide ion transport with respect to anisotropic strain in ceria lattice. (C) 2012 Elsevier B.V. All rights reserved.