Hydrolysis including the cell disintegration is the rate-limiting step of anaerobic digestion of waste activated sludge. Adding Fe(Ⅲ) oxides to enrich the Fe(Ⅲ)-reducing bacteria via the dissimilatory iron reduction can enhance the decomposition of complex organic compounds, which is considered as an effective method to promote the hydrolysis and cell disintegration of waste activated sludge. However, the continuous addition of Fe(Ⅲ) oxides is uneconomical in the practical engineering. Adding nitrite, the intermediates in the denitrification, has been reported to promote the hydrolysis and cell disintegration of waste activated sludge.In the meantime the Fe(Ⅱ)in the sludge can be chemically oxided to Fe(Ⅲ).However, there are few reports on the effects of nitrite induced Fe(Ⅱ) oxidation and Fe(Ⅲ) reduction processes on sludge hydrolysis and cell disintegration and subsequent anaerobic digestion.In this study, adding nitrite into the waste activated sludge caused the oxidation of Fe(Ⅱ) with an efficiency of 97.3%. The produced Fe(Ⅲ) further drove the dissimilatory iron reduction.Analysis of scanning electron microscope and three dimensional fluorescence spectrum indicated that the hydrolysis and cell disintegration of waste activated sludge were significantly enhanced in the nitrite-added group, accompanied by the release of large amounts of soluble organic compounds.Analysis of microbial community showed that,both denitrifying and Fe(Ⅲ)-reducing bacteria were significantly enriched in the nitrite-added group. After anaerobic digestion, the sludge reduction and accumulative methane production in the nitrite-added group were 21% and 86% higher than that in the control group without nitrite, respectively.