Carbon nanofibers with a diameter less than 100 nm were prepared by electrospinning. We investigated the effect of size on the microstructure, morphology, and electrochemical performances of the nanotibers. Specific area and pore volume increased as fiber diameter decreased. However, the structural parameters, such as the interlayer distance, microcrystallite size, and fraction of graphitization exhibited no clear correlation with fiber diameter. Electrochemical measurements revealed that the discharge capacity of the nanofibers carbonized at 800 degrees C with a diameter of 65 nm was 434 mAlt/g under a current density of 150 mA/g, which was approximately 52% higher than that of the carbon nanofibers with a diameter of 684 nm. The fibers exhibited favorable cycling stability, with a retention ratio of 90% after 100 cycles. Increasing carbonization temperature facilitated the structural development of the nanofibers, but caused a reduction in the specific surface area, pore volume, and the amount of functional groups on the fiber surface. As a result, the specific capacity of the carbon nanofibers reduced considerably. (C) The Author(s) 2015. Published by ECS. All rights reserved.