A multilayer film of Mg and Ni was prepared by dc/ac magnetron sputtering and annealed below 623 K in vacuum to obtain polycrystalline Mg2Ni thin films. The phase transformation during heating process and optical switching properties of the films were investigated. The influence of the original crystalline state of Mg2Ni films on optical switching properties such as transmission, optical band gaps and the cycle times was discussed. The indirect optical band gaps of the fully hydrogenated amorphous Mg2Ni films were estimated by linear extrapolation. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.