Abstract: 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 (0.6mg/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.