Activated sludge was monthly sampled from a saline sewage treatment plant of Hong Kong (China) during June 2007 to May 2008 to analyze the microbial community shift along with environmental variations using denaturing gradient gel electrophoresis of polymerase chain reaction amplified 16S rDNA fragments. Environmental changes resulted into a seasonal microbial community shift characterized by alterations in species number and abundance in the sewage treatment plant. Correspondence analysis and cluster analysis on community structure profile showed that the 12 monthly samples fell into four groups, which is in accordance with season changing in Hong Kong. Canonical correspondence analysis revealed that and posed more significant effects on community structure than total phosphorus and total nitrogen, respectively. Compared with sludge retention time, influent total phosphorus had an inverse effect on the community structure shift, and chemical oxygen demand and showed a similar effects. Results of this study may contribute to the development of new knowledge involving the microbial community shift in sewage treatment plants.