A gradient Na-beta ''-Al2O3 film was in-situ formed on the surface of alpha-Al2O3 ceramic substrate by a gas-solid reaction method and characterized by SEM, XRD, EPMA and contact angle test. The experimental results demonstrated that the thickness of the gradient film was greatly dependent on the reaction temperature. The surface modification of the alpha-Al2O3 substrate established the symmetry of the parts of sodium battery to be sealed, which significantly enhanced the sealing reliability of the sodium battery. The mechanism of the in-situ gradient film formation was also deduced and discussed. (c) 2013 Elsevier B.V. All rights reserved.