Start-up　characteristics　of　the　integrated　anoxic/oxic　(A/O)　biofilm　reactor　with　different　carrier　packing　rates　were　investigated　for　the　treatment　of　domestic　wastewater　with　low　content　ratio　of　C　to　N　using　polyurethane　foam　and　polypropylene　hollow　ring　as　biofilm　carriers,　respectively.　In　R1　system,　the　carrier　packing　rates　in　anoxic　and　oxic　zones　are　45%　and　20%　respectively,　and　in　system　R2,　those　are　60%　and　30%,　respectively.　The　results　show　that　the　start-up　periods　of　R1　and　R2　system　are　27　d　and　24　d,　respectively,　and　R1　is　more　appropriate　for　practical　application.　After　start-up,　the　content　of　biofilm　in　oxic　zones　of　R1　and　R2　system　are　87.8%　and　79.5%,　respectively,　which　makes　smaller　volume　of　settler　of　the　integrated　reactor　and　abolishment　of　sludge　return　possible　in　subsequent　running　period.　Packing　rate　of　60%　of　the　polyurethane　foam　in　anoxic　zones　is　more　conductive　to　the　utilization　of　organics　by　pro-denitrification　compared　to　that　of　45%.　Biofilm　formed　under　condition　of　fluidized　carriers　in　oxic　zones　has　better　capability　of　simultaneous　nitrification-denitrification　(SND)　and　suitability　for　hydraulic　load,　whereas　it　increases　the　start-up　period,　and　the　performance　of　SND　can　be　regarded　as　a　fuzzy　reflection　of　biofilm　formation;　biofilm　formed　under　condition　of　fixed　carriers　has　a　less　star-up　period,　whereas　the　biofilm　is　vulnerable　to　the　shock　of　hydraulic　load.