In the present study, an attempt has been made to evaluate the treatability of a combined tannery and domestic wastewater using lab-scale activated sludge reactor under extended aeration. Studies on the efficiency of the activated sludge reactor were carried out by varying the hydraulic retention time of 15, 20, 25 and 30 h at solids retention time of 8, 12 and 16 days with organic loading rate varying from 1.61 to 3.32 kg COD/m3/day. The results showed that at the end of 30 h HRT at 16 days SRT with a lower F/M ratio of 0.08 kg BOD/kg MLSS, the maximum reduction of COD was observed to be from 2020 to 175 mg/L. This study has clearly shown the potential applicability of aerobic biological process in treating the combined tannery and domestic wastewaters under extended aeration.