Two　biofilm　annular　reactors　(BAR)　in　series　were　adopted　in　the　study,　and　the　effects　of　chlorine　and　chlorine　dioxide　disinfection　on　nitrification　were　compared　by　analyzing　residue　chlorine,　NH4+-N,　NO2--N,　NO3--N,　DO,　HPC　and　AOB.　The　results　showed　that　chlorine　dioxide　was　more　effective　than　chlorine　in　enhancing　the　effluent　concentration　of　residue　chlorine　and　DO.　The　concentration　of　NH4+-N　was　droped　while　NO2--N　and　NO3--N　were　increased　during　the　stage　of　chlorine,　but　not　happening　during　the　stage　of　chlorine　dioxide.　Compared　with　equivalent　chorine　(0.6　mg/L),　the　chlorine　dioxide　was　more　effective　in　removing　suspended　and　boifilm　HPC　with　2.54　log　and　1.63　log　reductions,　respectively,　and　both　suspended　and　biofilm　AOB　with　1　log　reduction.　Chlorine　was　ineffective　in　controlling　AOB.　Therefore,　the　effect　of　chorine　dioxide　combined　with　chloramine　was　better　than　chorine　combined　with　chloramine　in　mitigating　nitrification　in　distribution　systems.