Solid-state method as well as hydrothermal treatment and postannealing method were applied to prepare porous carbons, using sodium carboxymethyl starch (abbr. CMS-Na) as carbon source. The as-prepared carbons were further heated with urea to produce nitrogen-doped carbons. The nitrogen content within carbons has crucial impact on the surface areas, pore structures and capacitive properties. The experimental results reveal that the capacitive performances of nitrogen-doped carbons by solid-state method are much better than those by hydrothermal treatment and postannealing method. Specific capacitance at the current density of 1 A g(-1) of the carbon-S-N-1:20 sample has improved up to 176.0 F g(-1) from 1472 F g(-1) (carbon-S-blank); the carbon-H-N-1:20 sample has dramatically improved up to 170.3 F g(-1) from 683 F g(-1) (carbon-H-blank). Furthermore, the carbon-S-N-1:20 sample exhibits better rate capability, cycling stability, and power density. (C) 2013 Elsevier B.V. All rights reserved.