To improve the photocatalytic performance, heterogeneous elements have been doped in the lattice of photocatalyts, but the lattice distortion induced by the doping has not been sufficiently discussed. We now demonstrate the effect of the lattice distortion of TiO2 on its photocatalytic performance. A lattice-distorted TiO2 film showed a significantly higher photocatalytic activity than a non-distorted TiO2 film. For the lattice-distorted TiO2 film, a gradient of the lattice parameter was formed, leading to the forming of potential slopes of the valence band maximum and conduction band minimum as confirmed by DFT calculations. The potential slopes promote effective separation of the electron-hole to improve the photocatalytic performance. Graphical Abstract [GRAPHICS] .