Many industrial & semi industrial countries manage to use SBR in order to industrial & municipal wastewater treatment. Simplicity, flexibility & being economical of this technical biological treatment, lead it to its significance in utilizing for treating of industrial & municipal wastewater.To study this procedure, two units of SBR reactors' cylinder made of poly glass with 20 liter volume are used. For seeding the reactors, artificial wastewater with COD of 600 mg/l & BOD of 600 mg/l is used. Reactors with different solid retention time (SRT) (5, 10, 15, 20 & 25 days) are operated.After providing the steady state in the reactors, during 8 months research, sampling and testing parameters such as COD, BOD, MLSS, MLVSS, MLNVSS, pH, DO, SVI, SOUR & F/M were done.The results show that SRT (5, 10, 15, 20 & 25 days) for 10 days duration is the best results for COD removal efficiency without existence of any bulking, pin point flocks and also foaming & dispersed growth difficulties. In this SRT duration, the amount of SVI has been up to 90 mg/l. This SRT system has been used in growth phase and the amount of F/M is 0.25 (1/day). In the SRT less than 5 days, inorganic material removal & sludge volume index efficiency (SVI=215 mg/l) due to voluminous foaming generation & also fine coagulation generation resulted from lowness MLSS and dispersed growth occurrence respectively was low and it which shows that the system was operated in log growth phase & the amount of F/M has been1.8 (1/day). Also in the SRT more than 25 days filamentous bulking phenomenon was aggravated and system efficiency due to bad sludge settle ability (SVI=215 mg/l) was low & because of over growth of filamentous bacteria, sludge can not be settled easily.With the increasing of SRT (more than 20 days), due to entrancing to declining phase and increase of inactive & old bacteria, the rate of oxygen consumption & SOUR reached to 7 (mgo2/hr/grVSS), however in a low SRT (5 days), due to entrancing log growth phase & increase of active & young bacteria, the rate of oxygen consumption increased & SOUR reached up to 22 (mgo2/hr/grVSS).