Abstract: Chemical disposal and toxic pollutants are always one of the main environmental pollutants of water sources distributed from industrial wastewaters. The most suitable method for treatment such these wastewaters is biological systems. Effective microorganisms variety obviously depend on the type of pollutant, system and environmental conditions. Identification of the most effective microorganism is necessary for determination of optimum conditions, controlling method and monitoring of bioreactor to access the maximum efficiency and improvement of operation. The objective of this study is identification of effective microorganisms in phenol treatment procedure in combined biological system of biofilter and activated sludge Some amount of biological sludge was provided from domestic wastewater treatment plant as a primary source of microbe and added to the designed reactor. Then samples were collected after growth of microbial mass. Phenol concentration and environmental condition (i.e., dissolved oxygen and pH) were stabilized after gradually adaptation of the system to phenol. All samples were collected by sterile glass container. These samples were cultured on enrichment media and identified by various differential tests. Identification results proved phenol degrader bacteria are aerobe, nonfrementer and of negative for glucose test. These isolated bacteria were Pseudomonas aerginosa, Pseudomonas alcaligenes, Moraxella sp., Acinetobacter sp. and Brevundiomonas vesicularis. All these biodegrader microorganisms used only phenol for the both carbon and energy source, since phenol, nitrogen and phosphorus were the only available substrate in this system. Phenol was degraded at completely aerobic condition and dissolved oxygen concentration was sufficient.