High-temperature pyrolyzed FeNx/C catalyst is one of the most promising nonprecious metal electrocatalysts for oxygen reduction reaction (ORR). However, it suffers from two challenging problems: insufficient ORR activity and unclear active site structure. Herein, we report a FeNx/C catalyst derived from poly-m-phenylenediamine (PmPDA-FeNx/C) that possesses high ORR activity (11.5 A g(-1) at 0.80 V vs RHE) and low H2O2 yield (<1%) in acid medium. The PmPDA-FeNx/C also exhibits high catalytic activity for both reduction and oxidation of H2O2. We further find that the ORR activity of PmPDA-FeNx/C is not sensitive to CO and NOx but can be suppressed significantly by halide ions (e.g., Cl-, F-, and Br-) and low valence state sulfur-containing species (e.g., SCN-, SO2, and H2S). This result reveals that the active sites of the FeNx/C catalyst contains Fe element (mainly as Fe-III at high potentials) in acid medium.