The deterioration mechanism of nickel metal-hybrid batteries for hybrid electric vehicles (HEVs) has been investigated. For HEV use, the deterioration of the batteries is caused by the degradation of the positive electrode which is influenced by the migration of the constituent elements of the negative electrode alloy as well as by the oxidation of the negative electrode. Based on the elucidated deterioration mechanism, the degradation of the positive electrode has been suppressed by employing a hydrogen-absorbing alloy with higher oxidation resistibility for the negative electrode, prolonging the cycle life of nickel metal-hydride batteries for HEVs.