Two　biofilm　annular　reactors(BAR)　in　series　were　investigated　to　compare　the　relative　efficacy　of　chlorine　and　chlorite　in　nitrification　by　analyzing　residue　chlorine,　NH4+-N,　NO2--N,　NO3--N,　DO,　HPC　and　AOB.　The　results　showed　that　the　concentration　of　NH4+-N　was　dropped　and　those　of　NO2--N　and　NO3--N　were　increased　during　the　stage　of　adding　chlorine.　However,　that　was　not　happened　by　adding　chlorite　because　of　enhancing　the　effluent　concentration　of　residue　chlorine　and　DO.　Consequently,　the　chlorite　was　more　effective　than　chlorine　in　mitigating　nitrification.　Compared　with　equivalent　chorine　(0.6　mg/L),　the　suspended　and　attached　HPC　of　the　first　BAR　reduced　2.58　log　and　2.86　log,　respectively,　both　suspended　and　attached　AOB　declined　1　log　using　the　chlorite.　Therefore,　the　effect　of　chlorite　is　better　than　chorine　combined　with　chloramine　in　mitigating　nitrification　in　distribution　systems.