The Mg 2 Ni (1−x) Mn x (x=0, 0.125, 0.25, 0.375) hydrogen storage alloys have been synthesized by mechanical alloying (MA). The effects of substitution of Mn for Ni on the phase composition and microstructures of Mg 2 Ni-type alloys have been investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD results of milled samples indicate that substitution of Mn for Ni could inhibit the formation of MgNi 2 phase and the solid solubility of Mn in Mg 2 Ni phase is low. The calculation of average crystallite size and microstrain of Mg 2 Ni (1−x) Mn x alloys milled for 8h shows that with the increase of substitution amount of Mn for Ni from x=0 to x=0.375, the average crystallite size decreases first from 12.5nm to 9.6nm and then increases from 9.6nm to 11.8nm with the minimum 9.6nm obtained at x=0.25, while the microstrain monotonously decreases from 1.39% to 0.82%. SEM has revealed that the sizes of subparticles that constitute the powder particles in the composition Mg 2 Ni 0.75 Mn 0.25 are much smaller than those in other compositions.