In situ sludge reduction of the oxic-settling-anaerobic (OSA) process was investigated in this study and microbial community diversity in the system was analyzed by using a PCR-denaturing gradient gel electrophoresis (DGGE) approach. Comparing to the conventional activated sludge method, the production of excessive sludge in the OSA process was shown to be efficiently reduced by 44-50 % and the organic loading rate was observed to have a slight impact on sludge yield. As demonstrated by the slight variation of Shannon diversity indices (SDI), the dominant microbial composition remained stable in the OSA sludge with the increase of Ns. About 63% of clones represented by predominant bands in the DGGE pattern were affiliated with the subclass of b-proteobacteria. A number of bacteria in the OSA process were phylogenetically related to uncultured bacteria isolated from enhanced biological phosphorus removal (EBPR) sludge.