Mechanical deformations with/without elemental substitutions have been carried out on Mg-Ni-based alloys to improve their hydriding properties. The alloys Mg-x at.%Ni (x = 33, 38, 43 and 50) with different nanometer-scale structures were successively synthesized by mechanical milling of Mg2Ni mixed with various amounts of additional Ni, and the relations between their structural and hydriding properties were investigated in detail. By milling only Mg2Ni, nanostructured Mg2Ni composed of the intra- and inter-grain regions was obtained (x = 33). Amorphous MgNi (a-MgNi) as a third region was dispersed around the nanostructured Mg2Ni by milling of Mg2Ni mixed with additional Ni (x = 38 and 43), and the pure amorphous MgNi was homogeneously synthesized in equivalent composition (x = 50). The coordination numbers and interatomic distances of the Mg-D and Ni-D correlations in the amorphous MgNi-D show that deuterium occupies the interstitial tetrahedral site composed of nearly [2Mg2Ni]. According to the specific structural properties, there is obvious miscibility gap (plateau) pressure higher than 1 × 10-4 MPa at room temperature even in the amorphous phase, and the total hydrogen content reached up to 2.2 mass%. Further studies on an amorphous MgNi-based system were carried out to clarify the effect of the short-range ordering on the structural and hydriding properties due to substitution of Mg- and Ni-sites by various elements.