Nitrogen-doped graphene sheets (NGS) were obtained using a simple hydrothermal reaction with graphene oxide and hexamethylenetetramine (HMT). HMT plays important roles in both reducing graphene oxide and doping nitrogen into the graphene sheets. The prepared nitrogen-doped graphene sheets reduced by hexamethylenetetramine (NGS-HMT) have a high nitrogen (8.62atom%) content, in which various nitrogen species, such as pyridinic-N, pyrrolic-N, and quaternary-N, are detected. For supercapacitor applications, NGS-HMT shows a value of 161Fg−1 at a current density of 0.5Ag−1, and it also exhibits a high cycle stability.