As one of the most promising candidates to replace Pt-based electrocatalysts for oxygen reduction reaction (ORR) in proton exchange membrane fuel cell (PEMFC), iron-nitrogen-carbon (Fe-N-C) materials have attracted tremendous research interests in recent years. Different from platinum-group-metal (PGM) catalysts, Fe-N-C have several categories of active sites: Fe-Nx has 4-electron pathway; pyrrolic-N and hydrogenated pyridinic-N can partially reduce oxygen into peroxide; pyridinic-N can reduce peroxide into water. Due to this complicated environment and numerous synthesis approaches from various precursors, the electrocatalysis mechanism of Fe-N-C still needs intensive investigation. Here we report the proton independence/dependence of rate-determining step (RDS) for a specific Fe-N-C catalyst via kinetic isotope effect (KIE), in addition to the activity inhibition of proton-involved N active sites via two molecular inhibitors. It is found that Fe-N-C can behave proton independent RDS like conventional Pt electrocatalyst in kinetic-controlled region, but it undergoes proton-coupled electron transfer in mass transport-controlled region, probably attributed to N active sites. It is also discovered that two molecules, tris(hydroxymethyl)aminomethane and etidronic acid, can inhibit and transform two categories of N active sites respectively: tris can effectively poison Fe-Nx active sites and pyridinic-N, resulting in obvious increase of peroxide yielding and drop of half-wave potential; etridonic acid can readily inhibit pyrrolic-N as well as hydrogenated pyridinic-N but no damage to Fe-Nx active sites, leading to decrease of peroxide yielding and no change of half-wave potentials. These results elucidate that Fe-N-C catalysts could, in principle, compete with Pt-based ORR catalysts through careful optimization of structure and morphology.