Microporous carbon nanofibers were prepared by electrospinning from resole-type phenolic resin, followed by one-step activation. KOH was utilized to tune the fiber diameter and improve porous texture. By adjusting KOH content in the spinning solution, the fiber diameter could be controlled in the range of 252-666 nm and the microporous volume and specific surface area could be greatly improved. The electrochemical measurements in 6 M KOH aqueous solution showed that the microporous carbon nanofibers possessed high specific capacitance, considerable rate performance, and superior specific surface capacitance to conventional microporous carbons. The maximal specific capacitance of 256 F g(-1) and high specific surface capacitance of 0.51 F m(-2) were achieved at 0.2 A g(-1). Furthermore, the specific capacitance could still remain 170 F g(-1) at 20 A g(-1) with the retention of 67%. Analysis showed that the high specific surface capacitance of the resultant carbons was mainly attributed to optimized pore size (0.7-1.2 nm) and the excellent rate performance should be principally due to the reduced ion transportation distance derived from the nanometer-scaled fibers. (C) 2012 Elsevier Ltd. All rights reserved.