The effects of Ni and Co addition on the hydrogen storage properties of Mg 3Mm alloy was studied by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX) and pressure-composition isotherm (PCI) measurement. The hydrogen absorption kinetics and the thermodynamic parameters (apparent Δ H, Δ S) for Mg 3Mm dehydrogenation reactions in Mg 3Mm, Mg 3MmNi 0.1 and Mg 3MmNi 0.1Co 0.1 alloys have been also investigated. The maximum hydrogen storage content of Mg 3Mm, Mg 3MmNi 0.1 and Mg 3MmNi 0.1Co 0.1 alloys was improved due to that the addition of Ni and/or Co further spurred the MmH 3 phase transforming to MmH 2 phase. On the other side, the kinetics curves show the addition of Co could enhance hydrogen absorption rate while the addition of Ni change the hydrogenation reaction mechanism.