A　52L　pilot-scale　biological　nutrients　removal　system,　based　on　anaerobic/anoxic/oxic　(A2O)　configuration,　was　used　to　investigate　the　sludge　bulking　and　control　when　treating　synthetic　wastewater.　Good　biological　nutrients　removal　performance　could　be　achieved　when　the　average　DO　was　1.08　mg/L　in　the　oxic　zone　and　the　removal　efficiency　of　COD,　NH4+-N,　TN　and　PO43--P　were　86.8%,　97.5%,　86.5%　and　95.5%,　respectively.　But　sludge　settle　characteristics　deteriorated　during　this　period　and　SVI　increased　from　130.1　mL/g　to　265.8　mL/g,　causing　severe　sludge　bulking.　When　average　DO　concentration　inoxic　zone　from　increased　1.08　mg/L　to　2.16　mg/L,　the　sludge　settle　characteristics　was　improved　but　the　SVI　was　still　around　200　mL/g.　To　further　control　sludge　bulking,　a　bypass　flow　(15%　of　influent　flow　rate)　was　induced　to　the　beginning　of　oxic　zone.　The　sludge　bulking　was　controlled　and　the　SVI　was　about　100　mL/g　after　30　days　operation.　Low　organic　loading　rather　than　DO　concentration　in　the　oxic　zone　was　the　major　factor　for　sludge　bulking　and　the　appropriate　distribution　of　organic　loading　along　the　tank　was　the　best　way　to　control　sludge　bulking　in　A2O　process.