A new biological nutrient removal system combining A2/O process with fixed film was developed in this work and the characteristics of denitrifcation were especially investigated in the combined fixed film reactor(CFFR). Media was added in the anaerobic, anoxic and aerobic reactors, respectively. Tests were made to establish the effluent level of NOx-N, COD, DO and nitrite effects on NOx-N removal in the CFFR by decreasing hydraulic retention time (HRT) from 10.0 to 3.5 hours and by increasing internal recycle ratio form 0% to 200%. The influent was synthesized to levels similar to the average influent of municipal wastewater treatment plants in Korea. SARAN media with a porosity of 96.3% was packed 40%/30%/25% based on its reactor volume, respectively. It was found that COD rarely limited denitrification in the anoxic reactor because of high C/NOx-N ratio in the anoxic reactor, while DO concentration in the anoxic reactor and NO2-N/NOx-N from the aerobic effluent inhibited denitrifcation in the anoxic reactor. It was proved that the critical points of DO concentration in the anoxic reactor and NO2-N/NOx-N from the aerobic effluent were 0.15㎎/L and 10%, respectively. As the internal recycle ratio increased, DO concentration in the anoxic reactor and NO2-N/NOx-N from the aerobic effluent increased. Especially, at the condition of internal recycle ratio, 200%, DO concentration in the anoxic reactor and NO2-N/NOx-N from the aerobic effluent exceeded the critical points of 0.15㎎/L and 10%, respectively. Then, denitrification efficiency considerably decreased. Consequently, it was represented that the control of DO concentration in the anoxic reactor and NO2-N/NOx-N from the aerobic effluent can assure effective denitrifcation.