In　order　to　improve　nitrogen　removal　in　anoxic/oxic(A/O)　process　effectively　for　treating　domestic　wastewaters,　the　influence　factors,　DO(dissolved　oxygen),　nitrate　recirculation,　sludge　recycle,　SRT(solids　residence　time),　influent　COD/TN　and　HRT(hydraulic　retention　time)　were　studied.　Results　indicated　that　it　was　possible　to　increase　nitrogen　removal　by　using　corresponding　control　strategies,　such　as,　adjusting　the　DO　set　point　according　to　effluent　ammonia　concentration;　manipulating　nitrate　recirculation　flow　according　to　nitrate　concentration　at　the　end　of　anoxic　zone.　Based　on　the　experiments　results,　a　knowledge-based　approach　for　supervision　of　the　nitrogen　removal　problems　was　considered,　and　decision　trees　for　diagnosing　nitrification　and　denitrification　problems　were　built　and　successfully　applied　to　A/O　process.