The specific discharge capacity of γ-MnO2 was improved with chemical oxidation and vacuum-drying processes. The intergrowth structure and electrochemical performance were studied by X-ray diffraction and electrochemical measurements; the structure–electrochemical property relationships were clarified. Vacuum-dried γ-MnO2 showed structural changes with capacity fade as the temperature increased, while the chemically oxidized γ-MnO2 showed no structural change without any capacity fading. The latter material showed a first discharge capacity of 275mAhg−1, and a reversible capacity of 250mAhg−1 with two discharge plateaux after second cycle, which is the highest capacity among the γ-MnO2 prepared previously.