The　mass　balance　of　COD.　N　and　P　in　the　wastewater　treatment　process　was　studied　respectively　on　a　trial　basis　by　a　pilotscale　A(2)/O　system.　Under　three　kinds　of　operating　conditions　which　with　different　temperature　(14　degrees　C,　18　degrees　C,　21　degrees　C)　and　mixed　liquor　internal　recycle　ratio　(100%,　200%,　and　300%),　the　flow　and　distribution　of　COD.　N　and　P　in　the　system　were　tested　and　analyzed.　The　variation　of　these　three　indexes　in　anaerobic,　anoxic　and　aerobic　phases　of　the　treatment　process　can　be　directly　represented　by　the　experiment　results,　and　it　will　give　real　support　to　optimizing　the　running　condition　of　A(2)/O　system.