A 375nm thick LaMg2Ni alloy thin film was prepared by electron beam evaporation using LaNi, Mg and Ni targets, and followed by vacuum annealing at 350℃. Upon hydrogen loading/unloading, the reversible conversion from a metallic, reflecting state to a semiconducting, color-neutral transparent state was observed in the as-prepared film. The contrast ratios in both the transmission (400—900nm) and the resistivity between the reflecting state and the transparent state are over four orders of magnitude. The surface and the cross-section of the alloy films before and after hydrogenation were observed by scanning electron microscope and atomic force microscope, which showed that the lower reflection observed in the dehydrogenated film is due to the rough surface originating from the first hydrogenation.