The performance of simultaneous nitrogen and phosphorus removal with an inner loop sequencing batch reactor were investigated by simulated municipal wastewater. The experimental results showed that COD, NH4(+) -N, and TP can be removed efficiently after four hours aeration, during which dissolved oxygen concentration was at 6 mg/L at the beginning of aerobic phase and pH was in the rang of 7 - 8. The COD and NH4+ -N as well as TP concentration in the effluent were about at 4 - 48 mg x L(1), 0 - 2.0 mg x L(-1), and 0 - 1.4 mg x L(-1) respectively, which indicated the removal rate for each item were about 89.7% +/- 6.5%, 97.4% +/- 3.6%, 95.6% +/- 4.4% when the concentration of influent were about 170 - 260 mg x L(-1), 20 - 30 mg- L(-1), 8 - 20 mg L(-1), respectively. The removal rate of TIN( NH4 -N + NO3(-) -N + NO2(-) -N) was also reached about 70%. It was found during the research process that phosphorus removal can be achieved without anaerobic phase, which was conventionally considered as a key phase for phosphorus removal, and this phenomena can not be explained by traditional theory.