To improve the electrochemical performance of graphene materials, nitrogen-doped graphene sheets (NGS) were simultaneously reduced and functionalized with nitrogen (N) doping from graphene oxide (GO) by a simple process using 1 wt.% ammonia water solution as the reducing agent, nitrogen precursor and solvent. The NGS were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy-energy dispersive spectroscopy microanalysis, and differential scanning calorimetry. The thermal stability of NGS was much higher than that of GO. The N content in NGS was 4.4 at.% and a maximum specific capacitance up to 233.3 F g(-1) was obtained at 0.5 A g(-1). At 0.02 V s(-1), the NGS exhibited a specific capacitance of 140.3 F g(-1), which was over 8 times that of GO and nearly 2 times that of graphene without N-doping. These results revealed that N-doping of functional graphene provide remarkable improvements on the electrochemical capacitive performance of graphene materials. The NGS also showed high cycle stability of capacitive performance. (C) 2013 Elsevier B.V. All rights reserved.