Nanowire K0.19MnO2·0.2H2O is prepared by a hydrothermal method. It presents better electrochemical behavior than K0.45MnO2·0.3H2O from the solid-state method. The capacitance of nanowire K0.19MnO2·0.2H2O (148 F g−1) is much higher than that of K0.45MnO2·0.3H2O (132 F g−1). It presents excellent cycling performance even when the oxygen in the aqueous electrolyte is not removed, and there is no evident capacitance fading after 2500 cycles. The supercapacitor based on activated carbon and nanowire K0.19MnO2·0.2H2O delivers an energy density 41.3 Wh kg−1 (based on the total mass of the active electrode materials) at a power density of 156.8Wkg−1, higher than that based on activated carbon and K0.45MnO2·0.3H2O, 28.4W h kg−1 at a power density of 115.1Wkg−1. In combination with our former work, it suggests that both the nano structure and the surface area are important to the capacitance of KxMnO2 especially the latter.