Mg-based hydrogen storage alloys Mg 2 - x Ni 1 - y Ti x Mn y (0<x<1, 0<y<1) have been successfully synthesized using a diffusion method (DM). The structure of these alloys has been examined by X-ray diffraction. A new phase was found in the alloys. Multi-Rietveld analysis showed that the new phase has a cubic crystal structure and its chemical formula is A 3 BC 2 , in which A stands for Mg, C stands for Ni and B stands for some microstructure of Ti and Mn. Further studies reveal that the new phase has excellent gas-solid reaction properties and electrochemical capacities. In the solid-gas reaction the desorption capacity of the alloy was about 355.26 mlH 2 (S.T.P)g - 1 alloy. Its discharge capacity also reached values as high as 335.8 mAhg - 1 (50 mAg - 1 , -0.5 V vs. Hg/HgO electrode). Some possible reasons for the excellent performances have been suggested.