First-principle density functional theory (DFT) calculations are performed to study the active sites in FeN(4)G electrocatalysts, as well as ORR activity and mechanism. The possible intermediates and transition states existing in the possible reaction paths from Langmuir-Hinschelwood (LH) mechanism are investigated. The results show that the associative pathways of OOH* formation is prior to that of O-2* dissociation. The condition of proton adsorbed on top N sites (T2) is more beneficial to the reduction of O-contained species adsorbed on top Fe site (T1) compared to the conditions of proton adsorbed on top C sites (T3). However, the dissociation of O-2*, OOH* and H2O2* is more likely to occur on the paired T1-T3 sites, since their lower energy barriers compared to other paired sites. The most favorable four-electron reduction pathway follows the mechanisms: O-2* -> OOH*-> O*+H2O -> OH*+H2O -> 2H(2)O. The rate determining step for ORR on FeN(4)G is the reduction of O* into OH* (barrier, 0.47 eV). The most feasible pathway for ORR is downhill at a high electrode potential (0.76 V vs. SHE at pH = 0) according to the free energy diagram. Compared to the ideal catalyst, the adsorption energy of OOH* on FeN(4)G is much lower in free energy, while those of OH* and O* are slightly higher. Additionally, the elementary reaction rate for OOH*-> O*+H2O is much larger than that of OOH*-> H2O2 based on the parameter of activation barrier. Therefore, the formation of H2O2 (1) is unfavorable on FeN(4)G catalysts. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.