Membrane bioreactors (MBRs) are operated with the same principle of activated sludge (AS) except for solids separation; that it is achieved by filtration through membranes. This article aimed to study the effect of hydraulic retention time (HRT), solids retention time (SRT), and the mixed liquor suspended solids (MLSS) concentration on the performance of a hollow fiber (HF) submerged aerobic MBR with an area of 1.5 m2. A reactor of 200-liter volume was designed, and operated at HRTs of 4, 6, 8, and 10 hours. For each HRT, five SRT values were selected 5, 10, 15, 20, and 25 days. The MBR's operation cycle was 8 min filtration, 1 min backwashing and 1 min relaxation. Air was supplied continuously to maintain 1.5 – 2.0 mg/l of dissolved oxygen. Results of the experiments showed improvement of the removal efficiency for chemical oxygen demand (COD), total suspended solids (TSS), and ammonia nitrogen with increasing SRT and HRT. The highest removal efficiencies for COD, TSS, and ammonia nitrogen were 97.59%, 99.71%, and 90.54 % respectively, which were achieved at SRT of 25 d and HRT of 10 hrs. The study concluded that at MLSS < 10,000 mg/l, there was no concrete relationship between TSS removal efficiency and MLSS concentrations, but better removal efficiencies of TSS were obtained at MLSS > 10,000 mg/l. The relationship between MLSS and COD removal was clearer than that of TSS; the COD removal efficiency was improved as the MLSS increased; the higher removal efficiencies took place at MLSS > 10,900 mg/l.