Atomically dispersed iron-nitrogen-carbon (Fe-N-C) catalysts have emerged as the most promising alternative to the expensive Pt-based catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs),however suffer from low site density of active Fe-N4 moiety and limited mass transport during the catalytic reaction.To address these challenges,we report a three-dimensional (3D) metal-organic frameworks (MOF)-derived Fe-N-C single-atom catalyst.In this well-designed Fe-N-C catalyst,the micro-scale interconnected skeleton,the nano-scale ordered pores and the atomic-scale abundant carbon edge defects inside the skeleton significantly enhance the site density of active Fe-N4 moiety,thus improving the Fe utilization in the final catalyst.Moreover,the combination of the above mentioned micro-and nano-scale structures greatly facilitates the mass transport in the 3D Fe-N-C catalyst.Therefore,the multiscale engineered Fe-N-C single-atom catalyst achieves excellent ORR performance under acidic condition and affords a significantly enhanced current density and power density in PEMFC.Our findings may open new opportunities for the rational design of FeN-C catalysts through multiscale structural engineering.