: In This research a series of PSF membranes were prepared using Tetrahydroforan (THF), Dimethylformamide (DMF), N-methylpyrrolidone (NMP)and Dimethylacetamide (DMAC)solvents via phase inversion method . CO 2, CH 4, N 2 andO 2 gases were used to evaluate the gas separation properties of prepared membranes. Results of gas permeation are as followed: PSF-NMP PSF-DMAC PSF-THF PSF-DMF. Amoung these prepared membranes, PSF–DMF membrane has most CO 2 /CH 4 , CO 2 /N 2 and O 2 /N 2 ideal selectivities with amount of37.9, 13.34 and 5.5, respectively.In the next part of the study, the effect of propionic acid on gas separation properties of polysulfone membrane was investigated. To evaluate the effect of PA, 9 different solutions containing PA were prepared for membrane casting. Then the results were comprised to Robeson upper-bound,to select the optimum membrane. PSF membrane with 25%wt polysulfone and 35%wt propionic acid showed the best performance. The results showed that permeability and selectivity increase by increasing the amount of propionic acid in polysulfone membranes. Furthermore, when concentration of polymer in membrane structure was decreased, permeability of membrane was creased due to more molecular entanglements and restricted pathway and when concentration of polymer in membrane structure was decreased, permeability of membrane was increased because of non selective defects. At the end of the research the effect of Alumina nanoparticles on the gas separation properties of the PSF membrane was evaluated. Polysulfone and polysulfone-alumina nanocomposite membranes were prepared by thermal phase inversion method. The performances of prepared membrane in gas separation were measured at pressure of 10 barg and temperature of 35°C.Membranes were prepared with alumina content of 5, 10, 15, 20 and 30 wt%. The prepared nanocomposite membranes were characterized using ATR, SEM and XRD methods. The SEM results demonstrated good distribution of alumina particles in the polymer matrix. This confirms the desirable mixing of the alumina in polymer and also a good compatibility between two phases. Also the SEM results were showed the size of particles were nano scale and were not accumulated. From XRD patterns of nanocomposites membranes excluded that crystalinity of prepared samples decreased with nanoparticles. The FT-IR patterns show the absorption peak at 830, 1635 cm -1 respect to bonded hydroxyl for Alumina, that height of these picks was increased with nanoparticles. The gas separation results showed that permeability and selectivity of gases increase by increasing the amount of alumina in PSF-Alumina membranes. Permeability of CO 2, O 2 , N 2 and CH 4 was increased from 10.9, 6.38, 1.07 and 0.37 barrer in pure polysulfone to 15.31, 8.77, 1.38 and 0.46 barrer in the nano composite containing 30 wt% of the alumina nanoparticles respectively. Furthermore the selectivity of CO 2 /CH 4 , CO 2 /N 2 and O 2 /N 2 in the corresponding membranes demonstrated an increase from 29.46, 10.19, 5.96 in pure PSF to 33.28, 11.09 and 6.35 in the composite membranes. The CO 2 / N 2 and CO 2 / CH 4 selectivity were increase more than that of O 2 /N 2 by alumina content, resulting mainly from the superior solubility selectivity of polar CO 2 gas, while the solubility of O 2 /N 2 remains nearly constant. Keywords: membrane; polysulfone; alumina; nanocomposite; gas permeatio