The proton conduction of BaCe0.90Y0.10O3-delta (BCY) film on Al2O3 (0001) substrate grown by RF magnetron sputtering has been studied as a function of film thickness (20-600 nm) in the temperature region of 300-700 degrees C. The lattice constants and grain size of the as-deposited film increase with increasing flim thickness. The film thickness above similar to 200 nm, which close to the lattice constants of the bulk, shows high proton conductivity with small activation energy (E-A) of similar to 0.3 eV below 500 degrees C. The BCY thin film has the mixed valent states of Ce4+ and Ce3+ created by the charge transfer from 0 2p to Ce 4f states. The E-A corresponds to the energy difference between the top of valence band and the Fermi level. These results originate to the changes of O-O bond length and oxygen vacancies in the a-c plane of the BCY thin film. (C) 2014 Elsevier B.V. All rights reserved.