Abstract: In this study the influence of anhydrous lithium fluoride (LiF) and the novel modified microwave casting solution technique on membrane performance are investigated. The polyethersulfone (PES) asymmetric membrane was prepared by the dry/wet phase inversion process. In order to observe the effect of microwave dissolution technique on membrane performance, PES concentration in the dope solutions was kept to 20 wt.% through out the experiments. The concentration of lithium fluoride was varied from 1-5 wt.% and N, N-dimethylformamide (DMF) is used as the solvent. Viscosities of the dope solutions were measured and membrane performance was characterized in terms of pure water permeation (PWP), permeation rate (PR) and solute separation of PEG solutions of different molecular weights ranging from 600-35000 Da. The molecular weight cut off (MWCO), mean pore size and pore size distribution of the membranes were subsequently determined. Results disclosed that the addition of lithium fluoride to the PES/DMF solution and the microwave irradiation technique increases the membrane permeability while maintaining their separation properties. However, the LiF concentration was best kept to 2 wt.% concentration.