Abstract: Simultaneous nitrification and denitrification(SND) is the main reason to the high nitrogen removal of oxidation ditch process.To achieve SND at high efficiency,redox potential(ORP) was used as a controlling parameter.This paper finds that:1) U_ORP can indicate SND,if U_ORP is between-30 mV to 30 mV,SND progress favorably,but if U_ORP is more than 30 mV,nitrification can be produced enough,denitrification is low,if U_ORP is lower than-30 mV,the result is the reverse to U_ORP more than 30 mV.2) If the U_ORP is between-30 mV to 30 mV,the rate of total inorganic nitrogen is more than 88%,the NO^-3 being removed is about 99% of the total NO₃^- being removed.3) The logarithm of the ratio of NH₄⁺-N to NO₃^- is a linear correlation,R is -0.97.