Manganese oxide (MnO2) and boron dual-decorated carbon nanofibers with hierarchical micro/mesopores (PPMMABMn) were fabricated by electrospinning and subsequent thermal technique to study the effect of the MnCl2 content on the microstructures and electrochemical properties in application as an electrode material of electrochemical capacitors (ECs). Transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), mapping images, and X-ray photoelectron spectroscopy (XPS) confirm the compositions and amounts of boron and MnO2 in the CNF composites. Furthermore, the nitrogen sorption isotherms and the Brunauer-Emmett-Teller (BET) theory reveal the co-existence of micropores and mesopores, with the latter representing more than 50% of the pore volume fraction. A high MnO(2 )content and many mesopores in PPMMABMn composites contribute to the main redox pseudocapacitance of MnO2 and low internal resistance for ion charge diffusion for good capacitive performance in KOH aqueous electrolyte in terms of large specific capacitance, great energy density and high rate capability.