In order to investigate the relationship among phosphorus content of sludge, filamentous bacteria and sludge settleability, two sets of Sequencing Batch Reactor (SBR) inoculated activated sludge that had different dominant filamentous bacteria and fed with synthetic wastewater were operated in A2/O process. Phosphorus concentration in influent was increased from 10 mg/L to 30 mg/L. Result showed that with increase of phosphorus in influent from 10 mg/L to 20 mg/L and 30 mg/L, phosphorus content of the sludge in reactor No. 1 was raised from 2.5% at early stage of inoculation to 8.17% and 9.23% correspondingly. SVI increased rapidly from 110 mL/g to 300 mL/g around and were about 135-150 mL/g subsequently. It is considered that improvement of settlement of sludge in the reactor No.1 was contributed to certain extent to the increase of phosphorus content in the sludge to 8% above. While in reactor No. 2 phosphorus content in its sludge was only raised from 1.89% to 6.77% and 6.95% correspondingly with increase of phosphorus in influent from 10 mg/L to 20 mg/L and 30 mg/L. SVI of the sludge increased from 138 mL/g to 190-320 mL/g, and finally kept at 280 mL/g to 300 mL/g. Settlement of sludge in the reactor No.2 wasn t improved by increase of the phosphorus content in sludge. The different performance of sludge in settlement between the two reactors is considered to due to existence of S. natans in the reactor No. 1 that was stimulated to conduct biological phosphorus removal resulting in an increase of dense of the sludge. In contrast, it was not so in the reactor No. 2. Whether or not sludge bulking can be controlled by increase of phosphorus content in activated sludge depends on the succession of predominant filamentous bacteria.