Wastewater is the main point-source pollutant on a global scale. Between 90 and 95% of the sewage produced in the world is released into the environment without any treatment. ِDevelopment of petrochemical industry in IRAN, construction of new plants and up gradation of existing units, has lead to generate more wastewater of higher strength. Some kinds of aerobic and anaerobic processes are applied for treatment of this type of wastewater. Performances during startup of a anaerobic hybrid reactor (UASB/Filter) treating petrochemical wastewater at mesophilic condition were investigated. The reactor was seeded with flocculent sludge from a UASB plant treating dairy wastewater. The reactor was run for 5 months without adaptation but it was unsuccessful. Hence the adaptation of sludge considered in 2 stage operation