Abstract: The effect of the different P/C ratios,i.e.,2.9/100,1.4/100,0.57/100,0.29/100,1.4/100 and 2.9/100,on the dynamic changes of the phosphorus removal as well as the organism community was examined in an anaerobic/aerobic sequencing batch reactor(A/O-SBR) system,affiliated with Polymerase Chain Reaction-denaturing Gradient Gel Electrophoresis(PCR-DGGE).Results showed that when the P/C ratio dropped(2.9/100→1.4/100→0.57/100→0.29/100) gradually,the P-release to substrate-uptake ratios decreased companied by an increase in the intracellular glycogen content.Also,the DGGE profiles showed that with the decrease of the influent P/C ratios the dominant microbial species increased from 11 operation taxonomy units(OTUs) to 14 OTUs,and then dropped to 7 OTUs.Judged from the biochemical characteristics of the SBR,it can be confirmed that the predominant microbial populations in the SBR had changed from PAOs to PAOs and GAOs co-exist,and finally to GAOs.For the last recovery period,the P-release to substrate-uptake ratios increased gradually with the decrease of the intracellular glycogen content,indicating that PAOs can be re-cultivated through regulating the influent P/C ratio,once GAOs had dominated the EBPR system.While,the DGGE profiles showed that the dominant species of PAOs re-cultivated from the GAOs-dominated system was different greatly from both the GAOs-dominated system and the initial PAOs-dominated system.