This study investigated characteristics of a sequencing batch reactor (SBR) system which was varied with respect to sludge retention time (SRT) (5.9, 8.2, 10.5, 12.2, and 16.2 days). The removal efficiencies of chemical oxygen demand (COD) were more than 90% under all SRT conditions, and the greatest efficiency (92.2%) occurred with a SRT of 16.2 days. As the SRT increased, the denitrification rate per mixed liquor suspended solids (MLSS) during the anoxic(I) period decreased significantly from 166.3 mg NO(X)(-)-N/g MLSS d to 68.8 mg NO(X)(-)-N/g MLSS d. As the SRT increased, the phosphorus removal efficiency decreased from 47.1% (SRT of 5.9 days) to 31.0% for a SRT of 16.2 days, because active phosphate release and uptake occurred under shorter SRT conditions. The mass balance of nitrogen (with respect to nitrogen in the influent) at a SRT of 16.2 days (the highest nitrogen removal efficiency) showed 14.9% of nitrogen was removed in clarified water effluent, 49.7% was removed by the sludge waste process and 33.3% was removed by denitrification. Nitrogen processing was well accounted for in the SBR system as the nitrogen mass balance was close to 100% (97.9%).