Abstract: The influence of inner circulation on nitrogen and phosphorus removal was investigated using real wastewater in a modified pilot scale Orbal system, in which the volume of main reactor is 460 L. Excellent nutrient removal was achieved. The resistance of the system to ammonia load strike and the efficiency of TN removal was improved with the inner circulation increasing, while the phosphorus removal getting worse. Preferable nutrient removal was achieved when the inner circulation was control at 1. Mass balance in the system showed that about 0.18 mg P/mg COD was taken up by the microorganism, which was compared to literature. Denitrifying phosphate-accumulating organisms, of which the specific phosphorus uptake rate is 13.2mg P/(g VSS· h), were found to be existing in the system. The ratio of anoxic/aerobic phosphorus uptake rate indicates that the fraction of DNPAOs in total PAOs was 23%.