A gamma-MnO2 nanomaterial was synthesized by hydrothermally treating an amorphous manganese oxide, which was obtained by the reaction between NaMnO4 and MnSO4 at ambient conditions. It is only in a narrow range of adding an amount of NaOH (e.g., 0.08-0.09 M in our case) that gamma-MnO2 can be obtained. The addition of NaOH may affect the radii of hydrated Na+ and, consequently, the formation of gamma-MnO2; the related mechanism was discussed. gamma-MnO2 was characterized by X-ray diffraction (XRD), scanning electron microscope, transmission electron microscope, and temperature-programmed XRD. gamma-MnO2 was also characterized by the combined thermogravimetric/differential thermal analysis-mass spectrometry; accordingly, the annealed forms of gamma-MnO2 were prepared and characterized electrochemically. The results show that the gamma-MnO2 calcined at 275 degrees C for 10 h showed excellent capacity (296 mAh g(-1)) with a good rate capability. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3206586] All rights reserved.