In the present study, the effect of sludge retention time (SRT) on the performance of the membrane bioreactor (MBR) was investigated. For this purpose, an aerated activated sludge reactor equipped with submerged flat sheet membrane was operated at different SRT (10, 20, 30, and 60 days). The hydraulic retention time (HRT) and the organic loading rate (OLR) were kept constant at 7 h and 0.93 kg COD/m3.d, respectively. The mixed liquor suspended solids ranged from 4.5–7 g/L. To avoid the impact of variations in the characteristics of real wastewater, the lab-scale reactor was fed using synthetic wastewater. Available results indicated that increasing the SRT from 10 to 60 days improved the performance of the system as reflected in the COD removal. Also, complete nitrification has been reported at 60 days SRT. Moreover, when the system was operated at SRT of 30 and 60 days, the biomass was in the form of small flocs which increased reactor performance due to a decline in mass transfer resistance.