We developed a simple chemical method to obtain bulk quantities of N-doped, reduced graphene oxide (GO) sheets through thermal annealing of GO in ammonia. X-ray photoelectron spectroscopy (XPS) study of GO sheets annealed at various reaction temperatures reveals that N-doping occurs at a temperature as low as 300 degrees C, while the highest doping level of similar to 5% N is achieved at 500 degrees C. N-doping is accompanied by the reduction of GO with decreases in oxygen levels from similar to 28% in as-made GO down to similar to 2% in 1100 degrees C NH3 reacted GO. XPS analysis of the N binding configurations of doped GO finds pyridinic N in the doped samples, with increased quaternary N (N that replaced the carbon atoms in the graphene plane) in GO annealed at higher temperatures (>= 900 degrees C). Oxygen groups in GO were found responsible for reactions with NH3 and C-N bond formation. Prereduced GO with fewer oxygen groups by thermal annealing in H-2 exhibits greatly reduced reactivity with NH3 and a lower N-doping level. Electrical measurements of individual GO sheet devices demonstrate that GO annealed in NH3 exhibits higher conductivity than those annealed in H-2, suggesting more effective reduction of GO by annealing in NH3 than in H-2, consistent with XPS data. The N-doped reduced GO shows clearly n-type electron doping behavior with the Dirac point (DP) at negative gate voltages in three terminal devices. Our method could lead to the synthesis of bulk amounts of N-doped, reduced GO sheets useful for various practical applications.