Nitrogen　and　phosphorous　concentrations　of　effluent　water　must　be　taken　into　account　for　the　design　and　operation　of　wastewater　treatment　plants.　In　addition,　the　requirement　for　effluent　quality　is　becoming　strict.　Therefore,　intelligent　control　approaches　are　recently　required　in　removing　biological　nutrient.　In　this　study,　fuzzy　control　has　been　successfully　applied　to　improve　the　nitrogen　removal.　Experimental　results　showed　that　a　close　relationship　between　nitrate　concentration　and　oxidation-reduction　potential　(ORP)　at　the　end　of　anoxic　zone　was　found　for　anoxic/oxic　(A/O)　nitrogen　removal　process　treating　synthetic　wastewater.　ORP　can　be　used　as　online　fuzzy　control　parameter　of　nitrate　recirculation　and　external　carbon　addition.　The　established　fuzzy　logic　controller　that　includes　two　inputs　and　one　output　can　maintain　ORP　value　at　-86　mV　and　-90　mV　by　adjusting　the　nitrate　recirculation　flow　and　external　carbon　dosage　respectively　to　realize　the　optimal　control　of　nitrogen　removal,　improving　the　effluent　quality　and　reducing　the　operating　cost.