This study was conducted for the treatment of tannery wastewater and to develop simple design criteria under local conditions. BOD5, COD, total Cr, SO42-, S2-, SS, TDS and TS of the influent and effluent were measured to find process efficiency at various mixed liquor volatile suspended solids (MLVSS), dissolved oxygen (DO) and hydraulic detention time. Results of the study demonstrated that an efficiency of above parameters 93.0%, 92.5%, 94.9%, 62.6%, 98.2%, 87.9%, 82.1% and 82.4%, respectively, could be obtained if the activated sludge process (ASP) is operated at the MLVSS concentration of 3500-4500 mg/L, (DO) concentration of 4.1-5.5 mg/L keeping an aeration time of 12 h.