Nanosized Fe3O4-modified activated carbon composites for supercapacitor electrodes have been investigated. Structural and morphological characterizations of activated materials are carried out using X-ray diffraction and scanning electron microscopy, respectively. The electrochemical performances of the composite electrodes are evaluated by cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy. The experimental results show that the specific capacitances of the 10 wt % Fe3O4-modified activated carbon composite electrode (154.3 F g−1) is highly improved compared with that of Fe3O4 (78.5 F g−1) and AC (79.2 F g−1) at the current density of 5 mA cm−2, respectively. The charge/discharge tests show that it could retain 79.6% of its initial capacitance over 1000 cycles, suggesting its potential application for the fabrication of high-quality supercapacitors.