In this research the continuously up-flow pilot scale Moving Bed Biofilm Reactor (MBBR) which was consists of combined cylindrical anoxic/aerobic MBBR in nested form with anoxic/aerobic volume ratio equal to 0.16 under fully nitrification-denitrification process were used to treated 4 m 3 /day of domestic wastewater in Chongqing city at Southwest China. The treatment must be satisfactory to meet with grade B of discharge standard of pollutants for municipal wastewater treatment plant in China (GB/T18918-2002). Both the anoxic and aerobic reactors were filled to 50% (v/v) with Kaldnes (K1) biofilm carriers to attach and retain biomass. The reactors was operated under fully nitrification-denitrification process without sludge returning into the system and only an internal recycling was performed from aerobic to anoxic reactor. After developing the biofilm on the carriers, the effect of gas/water ratio on biological nutrients removal from domestic wastewater was investigated by operated the reactors under 5 different gas/water ratio ranging from 5/1 to 24/1. During this operation mode, the favorite internal recycle ratio and Hydraulic Residence Time (HRT) to eliminate nitrogen compounds were 100% of inflow rate and 6.2 h, respectively. The experiment results showed that optimum value of the gas/water ratio for simultaneous organic carbon and nutrients removal was equal to 7/1. In this gas/water ratio, the average removal efficiencies were 92.67, 99.12, 71.37 and 90.49% for COD, NH 4 + -N, TN and TP, respectively, while the average Dissolved Oxygen concentration (DO) in aerobic and anoxic MBBRs were 4.49 and 0.16 mg/L, respectively.