The effect of ion irradiation on the rate of the electrochemical hydrogen absorption by Mm–Ni based alloy was investigated. Since the low-energy ion irradiation is quite useful for surface modification of materials, it is expected that the hydriding properties of Mm samples were improved by ion irradiation. The research, which aimed at the improvement in occlusion ability of Mm alloy using ion irradiation, is the first trial in the world. In this study, the ion irradiation was made with H + , Ar + and K + in the acceleration energy range of 350keV, up to a dose of 1×10 16 cm −2 using Takasaki Ion Accelerators for Advanced Radiation Application (TIARA) in Japan Atomic Energy Research Institute (JAERI). The initial hydrogen absorption rate was increased as the ion dose was increased. These results clearly suggest that the ion irradiation induces a high concentration of vacancies that trap high concentration of hydrogen atoms at the initial stage of the reaction. The induction of a high concentration of vacancies in metal by the ion irradiation is found effective to enhance the initial activation in the electrochemical hydriding process of MmNi 5 .