To　solve　the　problem　of　slow　hydrolysis　rate　during　anaerobic　digestion,　20　kHz　of　ultrasound　and　energy　density　of　0.5　W/mL　were　applied　as　pretreatments　for　mesophilic　anaerobic　digestion　of　waste　activated　sludge.　The　sludge　was　digested　at　hydraulic　retention　time　(HRT)　of　20　d　after　sonication　of　0-100　min.　Results　showed　that　ultrasonic　pretreatment　was　beneficial　to　the　dissolution　of　soluble　chemical　oxygen　demand　(SCOD)　from　the　waste　activated　sludge,　coupled　with　the　rapid　degradation　in　the　dewatering　performance　of　the　sludge.　In　the　succeeding　anaerobic　digestion,　when　sonication　of　the　treated　sludge　was　more　than　80　min,　the　removal　rate　of　total　chemical　oxygen　demand　(TCOD)　and　the　sludge　reduction　rate　also　decreased　significantly.　Gas　and　methane　productions　were　highest　at　the　reactor　fed　with　sludge　sonicated　at　80　min.　At　this　condition,　the　content　of　CH4　was　more　than　53.8%,　and　the　average　yield　of　methane　was　about　36.2　mL/gVS.　Through　the　analysis　of　microbial　diversity,　we　observed　that　hydrolytic　and　acidification　bacteria　were　abundant　in　the　reactors.　Methanocorpusculum　and　Methanosaeta　were　the　alternating　dominant　methanogens　in　the　anaerobic　reactors,　with　adding　sludge　suffered　different　ultrasonic　time.　In　the　sludge　pretreated　with　ultrasonication　after　80　min,　the　relative　abundance　of　Methanocorpusculum　decreased　rapidly,　and　the　dominant　substrate　for　anaerobic　methanogenesis　changed　from　hydrogen　to　acetic　acid.　(C)　2017　Elsevier　B.V.　All　rights　reserved.