Synthetic azo dyes with aromatic rings have become an important source of water pollution worldwide due to its high color, high toxicity and non biodegradability. In this study, Microbial fuel cell coupled constructed wetlands (CW-MFCs) were constructed for azo dye degradation. The performance of X-3B degradation, COD removal and electricity generation in the CW-MFCs were investigated. Experimental results showed that COD removal rate with co-substrate was consistently higher than that of without co-substrate, with the removal rates of 76.2% and 51.7%, respectively. X-3B removal efficiencies with different hydraulic retention time (HRT) (24, 48 and 72 h) were 19.7%, 39.1% and 84.7% without co substrate. The highest detected power density was 0.043 W m(-3) at the external resistance of 600 Omega. UVvis absorption spectra analyze indicated that the degradation of reactive brilliant red X-3B first occurred at the conjugation system, and later at the naphthalene rings and the benzene rings. The best X 3B degradation effect was achieved with additional co-substrate and longer HRT. However, the extension of HRT may affect the internal resistance on the contrary.