The main problem associated with the treatment of high strength materials in an Anaerobic Baffled Reactor (ABR) is difficult breakdown of fat, protein and hydrocarbon molecules at early stage of anaerobic decomposition due to uncertain selection of HydraulicRetention Time (HRT) and therefore organic loading rate (OLR). Thus, in this study a modified four-compartment ABR with a working volume of 50 L was designed to determine the treatment efficiency and methane production rate of high strength wastewater. The first compartment in the ABR was doubled in size to provide longer solid retention time. Based on C/N ratio of 30, a mixture of 62 kitchen waste and 38 sewage sludge was used as substrate and fed to the reactor continuously. Initially the characteristics of kitchen waste were measured and the amounts of fat, protein cellulose, hemicellulose and lignin were found in proper level for anaerobic decomposition. Next the effects of different HRT and OLR were evaluated in the reactor. COD reduction and biogas production were investigated at different HRT (5, 4, 3 and 2 days) and OLR (2, 3, 4, 5 and 6 kg/m3d). Results show that the highest COD removals (74.5 and 75.4) were observed at 3 days HRT and OLR of 2 kg/m-3day, respectively. While the best production of biogas (7.40 and 9.10 L day-1) was observed at 5 days HRT and OLR of 6 kg/m3 day, respectively. © 2011 Asian Network for Scientific Information.