Nanosized poly(3,4-ethylenedioxythiophene)/manganese oxide (PEDOT/MnO2) composite was prepared by an interfacial polymerization method. The structure and morphology of the product was characterized by IR, XRD, BET surface area, SEM, and TEM techniques. The results showed that the as-prepared amorphous nanocomposite had a porous structure. The pore size was about 5-25 nm and the surface area reached 98 m2·g-1. Electrochemical properties of the PEDOT/MnO2 electrode were determined by cyclic voltammetry (CV), galvanostatic charge/discharge, and electrochemical impedance spectrometry (EIS). The results indicated that the specific capacitance of the PEDOT/MnO2 nanocomposite as a single electrode was 196.3 F·g-1 at a current density of 0.5 A·g-1 in 0.5 mol·L-1 Na2SO4 electrolyte within a potential range of -0.2 - 0.8 V(vs SCE). The capacity retention of the PEDOT/MnO2 electrode was about 90% after 500 cycles.