Rechargeable performance was realized in a Mg-battery using a β-Cu2Se cathode. The average specific capacity was approximately 120 mAh/g for the first 35 cycles at room temperature, which was comparable with that of Chevrel phase as a prototype cathode material for Mg-batteries. The specific capacity was improved up to about 230 mAh/g by downsizing β-Cu2Se crystals, the short side length of which is about ∼100nm. The displacement reaction, in which the inserted Mg ions induce the displacement and extrusion of Cu ions from Se-sublattices, was suggested as the plausible mechanism for the reversible electrochemical reaction in the battery cell cathode. This mechanism was supposed from the structural relationship between β-Cu2Se and MgSe.