Abstract: An all-vanadium redox-flow battery (VRB) typically uses carbon felt as the electrode material in order to enhance the reactive current. The carbon felt made by polyaerylonitrile-based polymers can be heat-treated to increase the number of surface oxide sites and to improve its crystallinity and conductivity. It was examined in VRB that the over potentiality was mainly caused by positive pole reactions. The heat treatment of positive felt could decrease the cell's resistance dramatically, but that of negative felt induced low hydrogen evolution over potentiality and low current efficiency. With raw felt as the negative pole material and heat-treated felt as the positive pole material the test cell could get the optimal energy efficiency.