Background:　Anaerobic　digestion　has　been　widely　applied　to　treat　the　waste　activated　sludge　from　biological　wastewater　treatment　and　produce　methane　for　biofuel,　which　has　been　one　of　the　most　efficient　solutions　to　both　energy　crisis　and　environmental　pollution　challenges.　Anaerobic　digestion　sludge　contains　highly　complex　microbial　communities,　which　play　crucial　roles　in　sludge　treatment.　However,　traditional　approaches　based　on　16S　rRNA　amplification　or　fluorescent　in　situ　hybridization　cannot　completely　reveal　the　whole　microbial　community　structure　due　to　the　extremely　high　complexity　of　the　involved　communities.　In　this　sense,　the　next-generation　high-throughput　sequencing　provides　a　powerful　tool　for　dissecting　microbial　community　structure　and　methane-producing　pathways　in　anaerobic　digestion.　Results:　In　this　work,　the　metagenomic　sequencing　was　used　to　characterize　microbial　community　structure　of　the　anaerobic　digestion　sludge　from　a　full-scale　municipal　wastewater　treatment　plant.　Over　3.0　gigabases　of　metagenomic　sequence　data　were　generated　with　the　Illumina　HiSeq　2000　platform.　Taxonomic　analysis　by　MG-RAST　server　indicated　that　overall　bacteria　were　dominant　(similar　to　93%)　whereas　a　considerable　abundance　of　archaea　(similar　to　6%)　were　also　detected　in　the　anaerobic　digestion　sludge.　The　most　abundant　bacterial　populations　were　found　to　be　Proteobacteria,　Firmicutes,　Bacteroidetes,　and　Actinobacteria.　Key　microorganisms　and　related　pathways　involved　in　methanogenesis　were　further　revealed.　The　dominant　proliferation　of　Methanosaeta　and　Methanosarcina,　together　with　the　functional　affiliation　of　enzymes-encoding　genes　(acetate　kinase　(AckA),　phosphate　acetyltransferase　(PTA),　and　acetyl-CoA　synthetase　(ACSS)),　suggested　that　the　acetoclastic　methanogenesis　is　the　dominant　methanogenesis　pathway　in　the　full-scale　anaerobic　digester.　Conclusions:　In　short,　the　metagenomic　sequencing　study　of　this　work　successfully　dissected　the　detail　microbial　community　structure　and　the　dominated　methane-producing　pathways　of　a　full-scale　anaerobic　digester.　The　knowledge　garnered　would　facilitate　to　develop　more　efficient　full-scale　anaerobic　digestion　systems　to　achieve　high-rate　waste　sludge　treatment　and　methane　production.