Extracellular　nitrite　has　been　used　to　improve　the　digestion　of　waste　activated　sludge　(WAS).　However,　the　underlying　mechanism　remains　largely　unknown.　In　this　study,　WAS　was　treated　with　0.2　gNO(2)(-)-N/gVSS　for　7　days　and　its　performance　was　compared　to　that　of　aerobic　and　anaerobic　treatments.　The　addition　of　nitrite　had　a　distinct　effect　on　the　reduction　of　VSS/SS　and　the　accumulation　of　soluble　organics　compared　to　the　control　reactors.　As　evident　by　the　variations　of　extracellular　polymeric　substances　(EPS),　nitrite　addition　had　a　positive　effect　on　decreasing　protein.　In　particular,　the　decrease　of　protein　mainly　occurred　in　tightly　bound　EPS　(TB-EPS),　which　caused　sludge　disintegration　and　enhanced　sludge　reduction.　Additionally,　the　decrease　of　microbial　diversity　with　nitrite　addition　was　significant　compared　to　the　control　reactors,　accompany　with　a　decrease　of　live/dead　cells　ratio　and　an　increase　of　supematant　DNA　concentration.　This　suggests　that　nitrite　could　cause　cell　death　and　lysis,　resulting　in　sludge　degradation.　Thus,　nitrite　addition　enhanced　sludge　treatment　through　the　combined　effect　of　TB-EPS　disintegration　and　cell　lysis.　These　findings　will　be　useful　for　the　optimization　of　sludge　treatment　process　based　on　nitrite　addition.