Abstract--- The present study evaluated the performance of UASBR through a laboratory model (25 liters of total volume) for treating the synthetic tannery effluent with maintained ambient psychrophilic temperature (20-28oC) at phase I and mesophilic temperature (30-40oC) at phase II. This model was studied its treatment efficiency in terms of COD reduction. In the phase I, the average varying influent COD applied over the model are 6520, 7083, 7515, 8113 and 8561 mg/l with flow rates for each average influent COD are 4.80, 9.60, 14.40, 19.20 and 24.00 l/d. It was found to be successful with COD removal of 80.54% for the operating conditions of OLR at 0.096kgCOD/kg VSS day, VLR at 1.45kgCOD/m3day and HRT at 5.21 days. In the phase II the average varying influent COD of 6515, 7027, 7571, 8104 and 8514 mg/l were applied with same flow rates. The experimental work on UASBR model is found successful with 82.60 % COD removal under the operating conditions of OLR at 0.091 kg COD/kg VSS day, VLR at 1.46 kg COD/m3day and HRT at 5.21 days. The reactor achieved BOD, TSS, TDS, N and P removal efficiency was observed at phase I and II are 82.8 %, 75.6 %, 77.2 %, 33 % & 56 % and 84.8 %, 76 %, 79.5 %, 35 % & 55 % respectively. The ratio of VFA to alkalinity was varied between 0.17-0.34 during the treatment.