Out-membrane cytochrome c(Cyt c) plays an important role carrying electrons from the inside of microbes to outside electron acceptors. However, the active sites of Cyt c are wrapped by nonconductive peptide chains, hindering direct extracellular electron transfer(EET). Humic acids(HA) have been previously proven to efficiently facilitate EET. However, the inherent mechanism of HAstimulated EET has not been well interpreted. Here, to probe the mechanism behind HA-stimulated EET, we studied the interaction between Cyt c and HA. The attachment of active in vivo Cyt c on a graphite electrode was achieved when MR-1 cells were self-assembled on the electrode surface. Pure horse-heart Cyt c was covalently immobilized on an electrode via 4-aminobenzoic acid to create an active in vitro Cyt c-enriched surface. Cyclic voltammetric measurements and scanning electron microscopy confirmed the immobilization of bacterial cells and pure Cyt c protein. Electrochemical methods revealed that HA could enhance the electrocatalytic current of both in vitro and in vivo Cyt c towards oxygen and thiosulfate, suggesting enhanced EET. The blue-shifted soret band in the UV-Vis spectra and changes in the excitation/emission matrix fluorescence spectra demonstrated that Cyt c interacted with HA to form organic complexes via electrostatic or hydrogen-bonding interactions. The results will help understand electron shuttle-stimulated EET and develop bacteriabased bioremediation and bioenergy technologies. Out-membrane cytochrome c (Cyt c) plays an important role carrying electrons from the inside of microbes to outside electron acceptors. However, the active sites of Cyt c are wrapped by nonconductive peptide chains, hindering direct extracellular electron transfer (EET). Humic However, the inherent mechanism of HAART stimulated EET has not been well interpreted. Here, to probe the mechanism behind HA-stimulated EET, we studied the interaction between Cyt c and HA. The attachment of active in vivo Cyt c on a graphite electrode was achieved when MR-1 cells were self-assembled on the electrode surface. Pure horse-heart Cyt c was covalently immobilized on an electrode via 4-aminobenzoic acid to create an active in vitro Cytotrmmetric measurements and scanning electron microscopy confirmed the immobilization of bacterial cells and pure Cyt c protein. Electrochemical methods revealed that HA could en hance the electrocatalytic current of both in vitro and in vivo Cyt c towards oxygen and thiosulfate, suggesting enhanced EET. The blue-shifted soret band in the UV-Vis spectra and changes in the excitation / emission matrix fluorescence spectra that that Cyt c interacted with HA to form organic complexes via electrostatic or hydrogen-bonding interactions. The results will help understand electron shuttle-stimulated EET and develop bacteria-based bioremediation and bioenergy technologies.