In　this　study,　the　ammonia　removal　and　stability　of　partial　nitrification　(PN)　were　investigated　in　an　activated　sludge　system　(R1)　and　a　biofilm　system　(R2).　The　two　systems　were　operated　with　identical　setup　and　seeded　with　same　sludge,　to　treat　the　sewage　with　ammonia　of　60-65　mg　L-1　at　14-16　degrees　C.　R1　performed　a　higher　ammonia　removal　ability　but　a　lower　stability,　while　R2　showed　a　contrary　result.　In　R1,　both　ammonia-oxidizing　bacteria　(AOB)　and　nitrite-oxidizing　bacteria　(NOB)　showed　higher　bioactivity,　AOB　and　Nitrospira　presented　lower　biodiversity　while　Nitrobacter　showed　higher　result　than　those　of　R2.　FISH　results　indicated　the　lower　proportion　of　AOB　in　R2,　and　relative　low　proportion　of　NOB　in　both　the　two　systems.　Activated　sludge　system　should　be　operated　as　half　PN　or　treat　high　ammonia　sewage　under　low　DO,　while　biofilm　system　could　run　as　totally　PN　or　treat　low　ammonia　sewage　with　high　DO.　(C)　2016　Elsevier　B.V.　All　rights　reserved.