A　full-scale　wastewater　treatment　plant　(WWTP)　with　three　separate　treatment　processes　was　selected　to　investigate　the　effects　of　seasonality　and　treatment　process　on　the　community　structures　of　ammonia-oxidizing　archaea　(AOA)　and　bacteria　(AOB).　And　then　DNA-based　stable　isotope　probing　(DNA-SIP)　was　applied　to　explore　the　active　ammonia　oxidizers.　The　results　of　high-throughput　sequencing　indicated　that　treatment　processes　varied　AOB　communities　rather　than　AOA　communities.　AOA　slightly　outnumbered　AOB　in　most　of　the　samples,　whose　abundance　was　significantly　correlated　with　temperature.　DNA-SIP　results　showed　that　the　majority　of　AOB　amoA　gene　was　labeled　by　C-13-substrate,　while　just　a　small　amount　of　AOA　amoA　gene　was　labeled.　As　revealed　by　high-throughput　sequencing　of　heavy　DNA,　Nitrosomonadaceae-like　AOB,　Nitrosomonas　sp.　NP1,　Nitrosomonas　oligotropha　and　Nitrosomonas　marina　were　the　active　AOB,　and　Nitrososphaera　viennensis　dominated　the　active　AOA.　The　results　indicated　that　AOB,　not　AOA,　dominated　active　ammonia　oxidation　in　the　test　WWTP.