Fe-N-C materials with Fe-N-x coordination sites are promising to replace the state-of-the-art Pt-based electrocatalysts for oxygen reduction reaction (ORR) due to high activity and low cost although there is still debate on its activity origin. Herein, we develop a strategy to investigate the role of iron species in Fe-N-C nanohybrids for catalysing ORR by delicately tuning the iron chemical state. By engineering peripheral substituents of iron porphyrin precursors, two Fe-N-C nanohybrids with metallic or oxidized iron species inside are achieved while holding the similar catalyst structure. Systematic experiments and theoretical analysis discover that metallic iron species promote the electrocatalytic activity of Fe-N-x sites for ORR, while oxidative ones inhibit O-2 adsorption on Fe-N-x sites, decreasing their ORR activity. This finding sheds light on unravelling the activity origin in Fe-N-C electrocatalysts for ORR towards their applications in energy devices.