Enhanced biological phosphorus removal (EBPR) process was operated in a laboratory-scale sequencing batch reactor (SBR) for one-month fed with acetate as the carbon source. The characteristic and the microbial population structure and space distribution dynamics of phosphate-accumulating organisms (PAOs) of start-up period were analyzed by fluorescent in situ hybridization (FISH). The relationship between enrichment of PAOs and phosphorus removal was discussed. PAOs could be enriched by recirculation activated sludge containing heterotrophs through anaerobic aerobic conditions. Portion of PAOs in the sludge increase from 11.5% to 40.48%. Bacteria population competition lasted 34 days. It started from PAOs replacing heterotrophs which cost 5 days then followed by 19 days intra-specific competition of PAOs. The last step was re-increasing of PAOs predominance. Phosphorus uptake by the enriched microbial community was not observed immediately. An accumulating-phase was necessary for PHA and poly-P storage. A lag-stage of 4-8 days existed when taking the performance of the reactor into consideration. Phosphorus removal by the predominant PAOs through intra-specific competition was achieved after accumulating-phase too. The FISH picture indicated that in the quickly growing phase PAOs cells were small and community structure was loose. The latter "accumulating-phase" cells became larger and the community structure clustered densely. This stage presented by better reactor performance.