Iron and nitrogen co-doped carbons show great potential for high-performance electrochemical oxygen reduction reaction. However, the rational design of atomically dispersed iron over nitrogen-doped carbons with activity comparable to that of Pt-C is still challenging. Herein, we develop a new approach that enables the direct formation of intrinsically nitrogen-functionalized two-dimensional sheet-like carbons containing a high concentration of single Fe atoms. This strategy only involves one-step pyrolysis of both, guanine and iron nitrate, without using any guiding agent and sacrificial template. The electrochemistry tests demonstrate an excellent ORR performance of the prepared Fe-N-x-C catalyst with a half-wave potential of 0.85 V and a limited current density of 6.5 mA/cm(2) in alkaline medium, outperforming the commercial Pt-C and most of previously reported Fe-N-x-C catalysts. We believe that the emergence of superior ORR performance is mostly attributed to the uniform dispersion of single Fe atoms at the molecular level and the formation of abundant coordinated Fe-N-x sites. In addition, the high surface area, optimal porosity and defective structure (particularly the defects at the edge) of the two-dimensional carbons are also beneficial for the improved ORR activity.