Nutrient removal from synthetic wastewater by sequencing batch operation was studied at different solids retention times (SRTs). The nutrient removal process was consisted of anaerobic, anoxic I, oxic I, anoxic II, oxic II and settling phases. Hydraulic residence times (HRT) of the aforementioned phases were kept constant at 2/1/4.5/1.5/1.5 h. Settling phase was 0.5 h for all experiments. Solids retention time was varied between 5 and 30 days at six different levels. Effects of SRT (sludge age) on COD, nitrogen (NH4-N, NO3-N) and phosphate (PO4-P) removal were investigated and the optimal sludge age resulting in maximum nutrient removal efficiency was determined. The highest COD (94%), NH4-N (84%) and PO4-P (70%) removal efficiencies were obtained at the sludge age of 10 days, although a sludge age of 15 days resulted in only slightly lower values. Sludge ages larger than 15 days resulted in lower nutrient removal efficiencies as compared to those obtained at 10 or 15 days of sludge age. Sludge volume index (SVI) was also minimum (55 ml/g) at sludge age of 10 days. MLSS concentration increased with sludge age resulting in MLSS concentration of 3500 mg/l at SRT of 30 days. On the basis of these results, a sludge age of 10 days was found to be optimal resulting in maximum nutrient removal efficiencies and minimum SVI.