Rechargeable performance was realized in a Mg-battery using a beta-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 beta-Cu2Se crystals, the short side length of which is about similar to 100 nm. 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 beta-Cu2Se and MgSe. (C) 2016 Elsevier Ltd. All rights reserved.