The objectives of this study were to evaluate the removal characteristics of total nitrogen, the influence factor of denitrification and the optimum operating condition in the pigment wastewater treatment using PAC-A/O process. The operating conditions of PAC-A/O process were mean BOD volumetric loading 0.86 kgBOD/m3/day, mean F/M ratio 0.072∼0.13 kgBOD/kgMLVSS/day and mean C/N ratio 3.47, respectively. The conditions of anoxic process in the field plant test were mean pH 8.3∼8.7 and mean temperature 34.1~44.0℃. The ORP bending point knee was eventually appeared in the ORP -107 mV and NO3 --N removal efficiency was increased according to the ORP decrease. In the ORP -107 mV below condition, the removal efficiency of T-N and NO3 --N was 92.3∼95.0% and 98.5∼99.7%. Denitrification rate was calculated to be 1.581∼1.791 mg NO3 --N/gMLSS/hr. The experimental results showed that the ORP control in the PAC-A/O process could be an effective method for treatment of pigment wastewater.