In order to improve the electrochemical hydrogen storage performances of the Mg2Ni-type alloys, Ni in the alloy was partially substituted by element Co. The nanocrystalline and amorphous Mg20Ni10-xCox (x=0, 1, 2, 3, 4) alloys were prepared by melt-spinning technology. The structures of the as-cast and spun alloys were studied by XRD, SEM and HRTEM. The electrochemical hydrogen storage characteristics of the alloys were measured. The results show that the substitution of Co for Ni leads to the formation of secondary phase MgCo2 without altering the major phase of Mg2Ni. No amorphous phase is detected in the as-spun alloy (x=0), whereas the as-spun alloy (x=4) holds a nanocrystalline and amorphous structure, confirming that the substitution of Co for Ni significantly increases the glass forming ability of the Mg2Ni-type alloy. The substitution of Co for Ni significantly improves the electrochemical hydrogen storage performances of the alloys, including the discharge capacity and the cycle stability, for which the increased glass forming ability by Co substitution is mainly responsible