The characteristics and mechanisms of enhanced electrochemical properties for a Mg,Ni alloy produced by a ball-milling treatment were investigated by electrochemical measurements and XRD analysis. Them particle size decreases, the internal strain increases, and the alloy gradually becomes amorphous on ball-milling. With the accompanying increased diffusion rate of hydrogen into the ball-milled Mg2Ni alloy, the limiting current density, i(erit), is decreased and the exchange current density, i(0), is increased. As a result, the specific discharge capacity is significantly increased compared to that of the as-received Mg2Ni alloy and reaches a maximum for a ball-milling time of 25 h.