Activated carbon fibers (ACFs) contain pores with a weak resistance to electrolyte migration but with high electrical resistance between the fibers. The ACFs used herein were prepared from ultra-thin polyacrylonitrile (PAN) fibers, to be used as electrodes in electrochemical double layer capacitors (EDLCs), by varying the activation temperatures and the holding times during steam activation. As the activation temperature and holding time were increased, the specific surface area increased along with the tance as high as 283 F g(-1) can be obtained using the ultrathin ACFs fabricated at 1000 C for 10 min with a specific surface area of 1408 m(2) g(-1). This investigation demonstrates that the surface area, pore structure, and surface functional groups of ACFs were all significant factors in determining the capacitive characteristics of ACFs. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 1615-1623, 2009