Nitrogen-doped graphene (NG) materials have been prepared by hydrothermal reaction of graphite oxide (GO) with urea, and their electrocatalytic properties towards oxygen reduction reaction in 0.1 M KOH are investigated. The introduction of urea leads to successful nitrogen insertion in the form of pyridinic, pyrrolic, and graphitic bonding configurations with enhanced reduction of GO, and an increase in the mass ratio between urea and GO gives higher nitrogen contents, which is accompanied by more defects in microstructure. Reduction of O-2 to OH- at lower overpotentials is favored by the incorporation of nitrogen, and the activity of NG is closely associated with nitrogen contents and microstructure. NG materials with ca. 7% nitrogen contents and moderate defect densities prepared from mass ratios of 1:200 and 1:300 (GO/urea) show the best performance. (C) 2012 Elsevier B.V. All rights reserved.