Nitric acid oxidation of electrospun carbon nanofibers, with varying degrees of oxidation, was carried out to understand the effect of acid treatment on the microstructure and capacitive behavior of the fibers. The increasing treatment time increased considerably the oxygen/carbon (O/C) ratio on the fiber surface, and the increase in the oxygen content was mainly contributed by the formation of carbonyl groups. However, nitric acid oxidation reduced the specific surface area and total pore volume of fibers. The specific capacitance of carbon nanofibers was closely related to the O/C ratio, and the specific capacitance of 365F/g for carbon nanofibers treated with nitric acid for 5h was obtained. This was ascribed to the increased fiber polarity that facilitated the diffusion of electrolyte ions into the nanofibers. Furthermore, the functional groups contributed to the pseudocapacitance, which was caused by faradaic reactions of oxygen-containing functional groups. The effect of these groups dominated that of structural change on the electrochemical performance of fibers.