The start-up and operational performance (total 212 days, including the start-up of 164 days) of an anaerobic baffled reactor (ABR), which is used to treat heavy oil produced water, was studied without the temperature control. Inoculums were mixtures of acclimated sediment taken from a heavy oil produced water treatment plant and digested sludge from a sewage wastewater treatment plant. The rod-shaped and spherical granules with colors of henna and black, in which Clostridia, Methanosarcina and Methanothrx sp. were main populations, were observed in each compartment of ABR after the reactor's successful start-up (day 164). Rhodopseudomonas with the activity of lipase and halotolerant, as a kind of photosynthetic bacteria, was also observed in the first five compartments. X-ray diffraction (XRD) showed that the spherical granule sludge was compact and contained a large amount of organics, amorphous materials, and crystals of Fe(2)O(3), FeS, and CaCO(3), whereas the rod-shaped granule sludge was incompact without crystals of Fe(2)O(3), FeS, and CaCO(3). Scanning electron microscope (SEM) showed that the skeleton construction of this rod-shaped granule was filamentous bacteria and amount of extracellular polymeric substances (EPS). The ABR, after successful start up, can achieve high average chemical oxygen demand (COD) and oil removals of 65% and 88% for heavy oil produced water with poor nutrient (COD:TN:TP, 1200:15:1) and high salt concentration (1.15-1.46%), respectively. Furthermore, ABR kept stable during 2.5 times the COD level shock load (0.50 kg COD m-3 d-1) for four days.