The　research　was　conducted　to　investigate　the　microbial　diversity　and　population　with　the　different　concentration　of　NH4+-N　in　a　biofilm　reactor　filled　with　volcanic　filter　for　completely　autotrophic　nitrogen-removal　over　nitrite　(CANON)　process.　The　reactor　had　an　excellent　performance　with　the　decreasing　of　NH4+-N　concentration　from　400　to　200　mg　L-1　while　NH4+-N　removal　loading　reduced　at　the　NH4+-N　concentration　of　100　mg　L-1.　Biodiversity　analysis　indicated　that　Nitrosomonas　related　aerobic　ammonia　oxidizing　bacteria　(AOB)　and　Planctomycetales-like　anaerobic　ammonia　oxidizing　(anammox)　bacteria　were　dominant　functional　bacteria.　Despite　the　different　influent　NH4+-N　concentration,　anammox　bacteria　had　a　low　and　stable　biodiversity,　which　was　not　the　same　to　AOB.　With　the　concentration　reduction　of　influent　NH4+-N,　the　estimates　of　total　bacteria　population　ranged　between　2.29　x　10(11)　and　1.44　x　10(12)　copies　mg(-1)　total　DNA,　and　the　quantity　of　AOB　decreased　while　anammox　bacteria　kept　stable.　The　population　of　Nitrospira　increased　and　little　Nitrobacter　was　detected　during　the　experiment.　(c)　2012　Elsevier　Ltd.　All　rights　reserved.