In　order　to　control　pre-denitrification　process,　variation　rules　of　DO,　ORP　and　pH　in　the　process　of　nitrification　and　denitrification　were　systematically　studied.　Feasibility　of　controlling　nitrification　and　denitrification　in　pre-denitrified　plants　using　DO,　pH　and　ORP　sensors　was　also　investigated.　Results　showed　that　the　variation　of　pH　in　anoxic　zones　could　be　classified　into　＂descending　type＂　and　＂rising　type＂,　which　indicated　if　the　denitrification　extent　and　nitrate　recirculation　flow　were　sufficient.　The　ORP　value　and　nitrate　concentration　at　the　end　of　anoxic　zone　also　had　good　correlation.　The　DO　concentration　in　the　first　aerobic　zone　could　indicate　the　influent　ammonia　load.　The　variation　of　pH　in　aerobic　zones　could　also　be　classified　into　＂descending　type＂　and　＂rising　type＂,　well　indicating　the　extent　of　nitrification,　aeration　and　alkalinity　were　sufficient.　The　experimental　results　also　showed　a　good　correlation　of　ORP　values　in　the　last　aerobic　zone　with　effluent　ammonia　and　nitrate　concentrations.　An　on-line　system　to　make　an　integrated　use　of　these　signals　for　on-line　control　of　aeration,　nitrate　recirculation　flow　and　external　carbon　dosage　was　presented　and　demonstrated　with　promising　results.