Porous carbon nanofibers with SiOC/SiOSi group (Si/ACNF) mats are produced by incorporating tetraethyl orthosilicate (TEOS) into polyacrylonitrile (PAN) via electrospinning/activation, and their electrochemical properties as a supercapacitor electrode are investigated. The Si/ACNF exhibits a high surface area of up to 1386.91m2 g−1 as result of the ultramicropore size distributions centered at approximately 0.8nm and mesopore volume fraction up to 38%. The gravimetric capacitance of the Si/ACNF webs in an organic electrolyte is 91.95Fg−1, and the energy densities are 59.72–88.23Whkg−1 over the power density range of 1000–20,000Wkg−1. The electrochemical capacitance of a Si/ACNF electrode is closely related to the pore size distribution, surface area, and ionic accessibility of the electrode material, providing a large charge capacity and high rate capability.