This study investigated the influence of the daily fluctuation of influent (IF) on nitrogen removal performance in oxidation ditch. The feasible velocity and dissolved oxygen conditions for simultaneous nitrification-denitrification (SND) were examined by conducting a trial experiment. The proportion of aerobic zone fraction (PAZF) was evaluated as a key control factor affecting the SND in the ditch. An innovative PAZF control technology was proposed to optimize the aeration conditions and the external recirculation rate of activated sludge in response to IF. The results indicated that the effluent quality effectively improved using the PAZF control technology. The effluent adhered to the new discharge standards for China: NH3-N < 5 and TN < 15 mg/L. The control technology was easily implemented by programmable logic controller at a wastewater treatment plant. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.