The Mg-Ni alloys with different Ni content were prepared by high-energy ball milling. Their structures were determined with X-ray diffraction and their discharge capacities as an electrode were tested. The results show that, as the Ni content increase from Mg2Ni, Mg1.5Ni to MgNi (the ratio of the atom), the structure of Mg-Ni alloy changed from nano-Mg2Ni, amorphous-MgNi to nano-MgNi. The Mg-Ni alloys with different structure exhibit different discharge capacities. As the increase of Ni content, the number of nano-MgNi increase and the discharge capacity could reached 500 mAh/g. As the milling time prolonged, the nano-MgNi with scarcely nano-Ni could be attained. As there was almost no nano-Ni, its discharge capacity is decreased.