Activated carbon fibers (ACFs) with super high surface area and well-developed small mesopores have been prepared by pyrolyzing polyacrylonitrile fibers and NaOH activation. Their capacitive performances at room and elevated temperatures are evaluated in electrochemical double layer capacitors (EDLCs) using ionic liquid (IL) electrolyte composed of lithium bis(trifluoromethane sulfone)imide (LiN(SO2CF3)2) and 2-oxazolidinone (C3H5NO2). The surface area of the ACF is as high as 3291m2 g−1. The pore volume of the carbon reaches 2.162cm3 g−1, of which 66.7% is the contribution of the small mesopores of 2–5nm. The unique microstructures enable the ACFs to have good compatibility with the IL electrolyte. The specific capacitance reaches 187Fg−1 at room temperature with good cycling and self-discharge performances. As the temperature increases to 60°C, the capacitance increases to 196Fg−1, and the rate capability is dramatically improved. Therefore, the ACF can be a promising electrode material for high-performance EDLCs.