The　entering　of　the　widespread　polyethylene　terephthalate　(PET)　microplastics　into　biological　wastewater　treatment　system　results　in　their　retention　in　sewage　sludge,　which　inevitably　enters　the　sludge　treatment　system.　However,　all　previous　studies　regarding　the　impact　of　microplastics　on　sludge　treatment　system　were　conducted　by　directly　adding　microplastics　to　system　and　focusing　on　anaerobic　sludge　digestion,　although　PET　microplastics　commonly　enter　into　the　biological　wastewater　treatment　system　first　before　sludge　being　subsequently　treated.　The　potential　impact　of　the　microplastics　on　waste　activated　sludge　(WAS)　aerobic　digestion　is　also　completely　missing.　Therefore,　herein　the　influences　of　PET　microplastics　with　different　entry　paths　on　WAS　aerobic　digestion　as　well　as　the　key　mechanisms　involved　was　firstly　explored.　Experimental　results　demonstrated　that　compared　to　the　control　test,　the　entering　of　PET　microplastics　to　biological　wastewater　treatment　system　inhibited　WAS　aerobic　digestion　by　10.9　+/-　0.1%　through　the　decreased　hydrolysis,　although　WAS　solubilization　during　aerobic　digestion　was　improved　due　to　the　change　of　generated　WAS　characteristics.　In　contrast,　when　PET　microplastics　was　directly　added　to　the　sludge　aerobic　digester,　there　was　little　impact　on　solubilization,　while　the　hydrolysis　were　inhibited　seriously,　thereby　suppressing　WAS　aerobic　digestion　more　severely　by　28.9　+/-　0.1%.　Further　investigation　revealed　that　PET　microplastics　reduced　the　populations　of　key　bacteria　(e.g.,　Saprospiraceae,　Chitinophagaceae　and　Xanthomonadaceae)　involved　in　aerobic　digestion　via　induced　oxidative　stress　or/and　releasing　toxic　chemical.　This　study　provided　a　more　accurate　approach　to　assessing　the　real　situation　regarding　the　influences　of　PET　microplastics　on　aerobic　sludge　digestion.　(c)　2020　Elsevier　Ltd.　All　rights　reserved.