In generally livestock wastewater contains high concentration of organic matter, nitrogen and phosphorus. Therefore, it is difficult to maintain good effluent quality by the conventional activated sludge process. The organic matter oxidation microorganism has a high yield value than the nitrification microorganism. Therefore if SRT gets shorter, it is hard to stabilization the nitrification microorganism. In this study, we used membrane bioreactor to make high MLSS and long SRT and operated additional nitrification reactor for advanced nitrification. So our study was performed on AO and AO(2) (Anoxic-Oxic-Oxic(Nitrification)) with HRT 5days. Before Nitrification Reactor combined to the AO system, we had feed high ammonia source to Nitrification Reactor for growing nitrification bacteria dominantly. AO(2) was composed with the 6L of Anoxic Reactor, 12L of Oxic Reactor and 9L of Nitrification Reactor. Operating system was studied under four steps according to internal recycle rate, changing HRT and adding the Nitrification Reactor. We used piggery wastewater which was centrifuged and biologically pretreated. The influents characteristics were CODcr 6.419 mg L-1, T-N 4.212 mg L-1, NH4+-N 2.560 mg L-1 and NO3--N 1050 mg L-1 so that was showed very low C/N ratio. In the AO system, internal recycle rate varied from 100% to 500%. We found out that internal recycle rate 300% achieved best efficiency. The result of total nitrogen removal, the total nitrogen effluent level of AO(2) system was 238 mg/L and attained about 94% removal efficiency, but effluent of AO system was 539 mg/L. So there was a 56% removal efficiency increase compared to AO. Especially, NH4+-N removal efficiency improved 68% and NO3--N removal efficiency had a 37% rise better than AO. The results suggest that combined nitrification reactor is able to achieve more efficient nitrogen removal without additional carbon source dosage in low C/N ratio.