Mg-based Laves phase alloys (Mg_0.7M_0.3)Ni₂, where M elements were Ca, La, Ce, Pr, Nd and Gd were successfully developed. The alloys can be synthesized by a conventional induction melting and annealing method from intermetallic compounds MgNi₂ and MNi₂ as a source of Mg, Ni and M elements. The crystal structures of the major phases of annealed (Mg_0.7M_0.3)Ni₂ alloys were C15-type for M=Ca and C15_b-type for M=La, Ce, Pr, Nd and Gd Laves structures, respectively. In the alloys except M=Ce, hydrogenation and dehydrogenation reversibly occurred. The maximum hydrogen contents of the alloys are 1.2–1.6 mass% (H/M:0.6–0.8) under hydrogen pressure of 5 MPa at 243–273 K. As the results of measurements of p-c isotherms, in the case of the M=Pr and Nd, two step plateaus were clearly observed. After a few hydrogenation/dehydrogenation cycles, the alloys kept C15-type or C15_b-type Laves structures without hydrogen-induced amorphization, disproportionation and decomposition.