The microstructure and hydrogenation property of hyper-eutectic Mg-Ni alloys were investigated. The specimens were Mg-31 mass%Ni, Mg-41 mass%Ni and Mg-50 mass%Ni alloys that are composed of primary Mg2Ni crystal and Mg-Mg2Ni eutectic structure. The amount of the primary Mg2Ni phase increased as the content of Ni increased. The hydrogenation behavior strongly depended on the Ni content and the number of the hydrogenation cycle, that is, as the increase of the Ni content in the specimen, the hydrogen absorption rate increased while the hydrogen absorption weight decreased. At the first cycle measurement, hydrogen absorption was slow for the Mg-31 mass%Ni alloy while hydrogen was rapidly absorbed for the Mg-50 mass%Ni alloy at 300°C. In all the specimens, saturation of the hydrogen absorption was achieved within a few minutes after the second cycle measurement. The maximum values of the hydrogen absorption weight decreased from 5.5 mass% to 4.1 mass% as the Ni content increased.