Efficient electron transfer among microbes,azo dye and anode is a key factor for simultaneous azo dye decolorization and electricity generation in microbial fuel cell (MFC).In this study,natural mediator-humic acid (HA) was introduced to enhance such extracellular electron transfer.The results showed that the addition of HA greatly enhanced azo dye decolorization and electricity generation simultaneously.Addition of 5g/L HA resulted in 45% increase in maximum power density and almost 300% increase in decolorization rate of Congo red.The data of reduction in anode polarization resistance and accelerated reductive decolorization of Congo red in the presence of HA indicated that HA is particularly effective in facilitate electrons transfer from microbes to azo dye and microbes to anode.Denaturing gradient gel electrophoresis and direct 16S ribosomal DNA gene nucleotide sequencing revealed that the addition of HA did not change the microbial community composition in the MFC but result in the growth of some dominant bacterial species.These species were most likely to utilize HA as mediator to accelerate the rate of electrons transfer involved in both electricity generation and azo dye decolorization.