New Mg-10wt%Ni hydrogen storage alloys were fabricated by casting which is a very simple and cost effective production process. Alloying elements such as Nb and Ti, which have relatively high melting temperatures and very low solubility in solid Mg, were successfully dissolved into the liquid Mg-Ni alloy. The Mg-Mg2Ni alloys contain a well-refined lamellar eutectic microstructure after solidification with a large interfacial area between the Mg and Mg2Ni phases which provides for good hydrogen sorption properties. This is considered to be due to the high diffusivity of hydrogen along the interphase boundaries. Addition of transition metals such as Nb and Ti results in the formation of intermetallic phases with a size about 10-20μm during solidification. Furthermore, Ti was found to be strongly segregated to the eutectic Mg-Mg2Ni interface. In the presence of Nb and Ti, the hydrogen sorption kinetics of the Mg-Mg2Ni alloy is further improved. This suggests that the transition metals act as active catalysts that eases and accelerates the hydrogen diffusion during hydrogenation and dehydrogenation. In this paper, we present the hydrogen storage properties and their relationship to the microstructure of the cast Mg-10wt%Ni alloys. Detailed microstructural analysis was carried out in order to further understand the hydrogen diffusion and storage mechanisms.