Background & Objectives: The membrane bioreactor (MBR) is one of the applications of membrane technology for wastewater treatment. A submerged MBR, in which membranes are directly submerged into the aeration tank, received more attention because of several advantages such as a better and more reliable effluent quality. Methods: In the present study, application of submerged membrane bioreactor in pilot scale for removal of chemical oxygen demand (average influent COD=230.6 mg/L, standard deviation=18.8 mg/L), total suspended solids (average influent TSS=104.2 mg/L, standard deviation=19.4 mg/L) and turbidity as Nephlometric Turbidity Unit (average influent turbidity=44.3 NTU, standard deviation=6.1 NTU) from raw influent waste water to Karaj wastewater treatment plant in different hydraulic retention time (4, 6, 8, 10 and 12 hours) was surveyed. Result: The results of this study showed that in 4 hours hydraulic retention time, the average effluent of COD, TSS and turbidity were 9.2 mg/L, less than 1 mg/L and 0.3 NTU, respectively. The mentioned parameters in 6 hours hydraulic retention time were 7.3 mg/L, less than 1 mg/L and 0.28 NTU, respectively. These parameters in 8 hours hydraulic retention time were 7 mg/L, less than 1 mg/L and 0.25 NTU, respectively. In 10 hours hydraulic retention time the mentioned parameters were 6.2 mg/L, less than 1 mg/L and 0.17 NTU, respectively. And in 12 hours hydraulic retention time were 4.3 mg/L, less than 1 mg/L and 0.16 NTU, respectively. Conclusion: This research showed that high treatment efficiencies of MBR for COD, TSS and turbidity at all operating conditions. Therefore, this technology could be regarded as a promising treatment stage for different applications of wastewater effluent reuse.