Large reversible capacitance and rapid rate capability are crucial to the realization of the manganese dioxide (MnO2) based electrode material but have been proved to be challenging to achieve due to the poor electronic conductivity of MnO2. Herein, the N-CNTs/MnO2 composites are prepared by using PPy-derived nitrogen-doped carbon nanotubes (N-CNTs) as the support frameworks to load nanosized MnO2. Benefitting from the high electronic conductivity of N-CNTs and the large pseudocapacitance of MnO2, the N-CNTs/MnO2-2 electrodes exhibit high specific capacitance of 366.5 F g(-1) at a current density of 0.5 A g(-1) which maintains 245.5 F g(-1) (67.0%) at 25 A g(-1), indicating excellent rate capability. Moreover, the as-fabricated asymmetric supercapaitors using N-CNTs/MnO2-2 and N-CNTs as the positive and negative electrodes achieve a wide stable operating voltage of 1.8 V and a high energy density of 20.9 Wh kg(-1), as well as outstanding cycling stability of 91.6% retention after 5000 cycles at a current density of 5 mA cm(-2). Therefore, these composites are promising electrode materials for the further high-power output energy storage and conversion devices.