Different　from　ammonia-oxidizing　bacteria　(AOB),　the　influence　of　ammonia　levels　on　ammonia-oxidizing　archaea　(AOA)　in　wastewater　treatment　systems　was　poorly　understood.　In　this　study,　sludge　contained　AOA　and　AOB　was　enriched　in　three　sequencing　batch　reactors　(named　as　R1,　R2,　and　R3)　under　different　ammonia　levels　(14,　56,　and　140mg/L)　using　polymerase　chain　reaction　(PCR),　cloning,　sequencing,　and　quantitative　real-time　PCR　(qPCR)　to　evaluate　the　effect　of　ammonia　levels　on　the　diversity,　abundance,　and　contribution　of　AOA　and　AOB　to　ammonia　oxidation.　Cloning　results　showed　that　there　was　only　one　dominant　AOA　species　(Nitrososphaera　cluster)　in　the　reactors　during　140-d　operation.　However,　AOB　communities　varied　significantly　among　the　reactors.　After　140-d　enrichment,　Nitrosomonas　ureae　cluster,　the　dominant　AOB　cluster　in　seed　sludge,　was　also　dominant　in　R1　and　R2,　while　Nitrosomonas　europaea　cluster　was　enriched　and　dominated　in　R3.　Diversity　of　AOB　was　higher　than　AOA　under　the　three　ammonia　levels.　Diversity　of　AOB　under　high　ammonia　concentration　(in　R3)　was　lower　than　that　under　low　ammonia　concentrations　(in　R1　and　R2).　QPCR　results　revealed　that　AOA　abundance　was　almost　unchanged　under　different　ammonia　levels　during　operational　period.　The　ratio　of　AOB　to　AOA　increased　from　3.68x10(2)　in　seed　sludge　to　4.90x10(3),　1.25x10(5),　and　3.77x10(5)　in　three　reactors　after　140-d　running.　This　study　suggested　that　AOB　was　much　more　competitive　than　AOA　in　high　ammonia　level　environments　and　probably　played　a　more　important　role　than　AOA　in　ammonia　oxidation.