To investigate the ion accumulation and diffusion behavior in micropores and mesopores, microporous and mesoporous carbon nanofibers were prepared from phenolic resin/polyvinyl alcohol and phenolic resin/polyvinyl alcohol/silica by electrospinning, followed by heat-treatment and removal of the template. Mesoporosity of carbon nanofibers was tuned from 3% to 83%. The structure and electrochemical properties of carbon nanofibers with different mesoporosity were compared. When mesopores were introduced, quantity of micropores was reduced, which led to a lower gravimetric capacitance and area-normalized capacitance. With the similar surface nature (oxygen content), micropores with pore sizes of 0.8-1.2 nm have higher capacitance per unit surface area (0.38 F m(-2)) than mesopores. The smaller separation of the electric double-layer derived from the distorted salvation shells in confined pores may be the reason for the enhanced capacitance. On the other hand, the mesoporous carbon nanofibers exhibited a better rate performance (C-20/C-0.1 = 89%) and much shorter relaxation time (as short as 0.082 s), which can be attributed to the fact that when mesopores were introduced into microporous carbons, in addition to the overall ion diffusion resistance being lowered, the ion diffusion distance in micropores was also reduced. (C) 2014 The Electrochemical Society. All rights reserved.