This　study　investigated　the　microbial　evolution　in　a　mainstream　partial　nitritation/anammox　(PN/A)　reactor　started　by　inoculation　from　sidestream　PN/A.　The　reactor　was　fed　with　pre-treated　sewage　and　operated　for　120　days　at　room　temperature　(24-26　degrees　C).　It　was　found　that　for　both　sidestream　and　mainstream　PN/A,　anammox　bacteria　preferentially　grew　in　granular　sludge　while　ammonium-oxidizing　bacteria　(AOB)　were　mainly　resided　in　flocculent　sludge.　After　120　days　operation,　the　abundance　of　anammox　bacteria　in　the　reactor　decreased　from　6.6　x　10(11)　to　3.2　x　10(11)　copies/L.　Besides,　a　shift　of　dominant　anammox　genera　from　Ca.　Brocadia　to　Ca.　Kuenenia　was　observed.　In　contrast,　the　dominant　genera　of　AOB　was　Nitrosomonas　throughout　the　operation.　Furthermore,　high-throughput　sequencing　revealed　that　heterotrophs　constitute　the　majority　of　microorganisms　in　PN/A　reactor.　Especially,　Chloroflexi,　which　can　utilize　cell　decay　materials　from　autotrophs,　were　enriched　under　mainstream　conditions.　This　study　provided　a　better　understanding　of　the　microorganisms　in　mainstream　PN/A　process.