Abstract: Effects of overcharging on the electrode material performance of MH/Ni batteries were investigated using SEM, XRD, TEM and EIS methods. The results indicated that the positive materials remained in their original spherical morphology after 70 cycles under normal charge/discharge processes. However, the spherical particles were cracked with the irreversible expansion of crystal lattice under overcharge conditions. Hydrogen storage alloy particles were covered with lots of needle-like corrosive products but without obvious pulverization after cycling. The main components of the needle-like corrosive products were detected to be rare earth hydroxide or oxide by means of EDAX. It was found from EIS analysis that R_s, R_t and Z_w increased to some extent, while C_i decreased. These changes during the overcharge cycling resulted in the degradation behavior of MH/Ni batteries.