Electronic and optical properties of Al-Ga codoped ZnO thin films were investigated by post-annealing. The lowest resistivity of the Al-Ga codoped ZnO films was observed from the 450 degrees C-annealed sample. The Fermi-level shift of the Al-Ga codoped ZnO film was similar to 0.6 eV from x-ray photoelectron spectroscopy, and the widening of optical-bandgap in the Al-Ga codoped ZnO film was similar to 0.3 eV. The correlations of optical-bandgap with Fermi-level shift and conduction band filling were suggested by schematic band diagrams. (C) 2011 Elsevier B. V. All rights reserved.