Characteristics of phosphorus removal bacteria were investigated by using three different types of electron acceptors, as well as the positive role of nitrite in phosphorus removal process. An (AO)2 SBR (anaerobic-aerobic-anoxic-aerobic sequencing batch reactor) was thereby employed to enrich denitrifying phosphorus removal bacteria for simultaneously removing phosphorus and nitrogen via anoxic phosphorus uptake. Ammonium oxidation was controlled at the first phase of the nitrification process. Nitrite-inhibition batch tests illustrated that nitrite was not an inhibitor to phosphorus uptake process, but served as an alternative electron acceptor to nitrate and oxygen if the concentration was under the inhibition level of 40mg NO2 - N · L- 1. It implied that in addition to the two well-accepted groups of phosphorus removal bacterium ( one can only utilize oxygen as electron acceptor, P1, while the other can use both oxygen and nitrate as electron acceptor, P2 ), a new group of phosphorus removal bacterium P3, which could use oxygen, nitrate and nitrite as electron acceptor to take up phosphorus were identified in the test system. To understand (AO)2 SBR sludge better, the relative population of the different bacteria in this system, plus another A/O SBR sludge (seed sludge) were respectively estimated by the phosphorus uptake batch tests with either oxygen or nitrate or nitrite as electron acceptor. The results demonstrated that phosphorus removal capability of (AO)2 SBR sludge had a little degradation after A/O sludge was cultivated in the (AO)2 mode over a long period of time. However, denitrifying phosphorus removal bacteria ( P2 and P3 ) was significantly enriched showed by the relative population of the three types of bacteria,which implied that energy for aeration and COD consumption could be reduced in theory.