Na-birnessite-type MnO2 (Na-OL-1) nanoplates and sodium manganese dioxide nanobelts with a 2×4 tunnel (Na-OMS-5) were synthesized by a hydrothermal method via one-pot oxidation reaction at 120 and 200°C, respectively. The microstructure, morphology, and thermal stability of the MnO2 products were characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, simultaneous thermogravimetric-mass spectrometry, and N2 adsorption–desorption measurements. Their electrochemical performance was investigated in order to understand the sodium ions intercalation−deintercalation processes in the lamellar or tunnel structure of MnO2. Na-OL-1 nanoplates exhibited a specific capacitance of 308 F g−1 at a current density of 0.2 A g−1, which is much higher than that of Na-OMS-5 nanobelts (141 F g−1). Moreover, the Na-OL-1 nanoplates showed a good rate capability and electrochemical stability, suggesting that it is a promising electrode material for supercapacitors.