The olefins such as ethylene and propylene are the basic material in the petrochemical industry. As the large amount of ethylene and propylene used as a feedstock in the production of plastics such as polyethylene and polypropylene. They play important roles in plastic production. Ethylene and propylene are manufactured by various methods which separating olefins from parrafins are inevitable. Currently, the separation is fulfilled by distillation, which is difficult and energy consuming. Polymeric membrane separations have been considered as the alternative methods because of low energy consumption and low cost investment. In the present study, the separation of ethylene/ethane and propylene/propane by polysulfone-silica nanocomposite membranes were studied. Silica nanoparticles were prepared via sol-gel method. Membranes were prepared by phase inversion method. The prepared membranes were characterized using FT-IR and SEM merhods. The results show a good distribution of silica particles in the polymer matrix and also a good compatibility between the two phases. The performances of the prepared membranes in ethylene/ ethane and propylene/ propane separation were measured at pressure of 2 bar and temperature of 30 ?C. The obtained results showed that the permeability of ethylene and propylene increased from 0.092 barrer and 0.054 barrer in pure polysulfone to 0.191 barrer and 0.149 barrer in the composite membrane containing 25%wt silica particles. The obtained results indicate that the selectivity of C 2 H 4 /C 2 H 6 and C 3 H 6 /C 3 H 8 increase from 2.18 and 2.93 in pure membrane to 2.62 and 3.58 in the composite membrane containing 25%wt silica particles. The diffusion and solubility coefficients were determined by the time lag method. The results solubility coefficient and decrease in diffusion coefficient of gases with increasing the silica. To investigate the effect of silica nanoparticles on plasticization phenomena, gas permeation of PSF and PSF-silica nanocomposite membranes was studied for ethylene, ethane, propylene and propane single gases at oressure range of 2-8 bar. Plasticization was obserevd in pure membrane at the feed pressure of 3 bar for all gases. In PSF-nanocomposite membranes with silica contents of 2.5, 5 and 10wt%, the plasticization pressure increase to 4, 5 and 6 bar for all gases but for PSF-nanocomposite membranes with silica contents of 20 and 25wt%, the plasticization pressure decreased to 5 and 4 bar and plasticization resistant did not improve. With increasing the feed pressure, the solubility coefficient of gases decreased and the diffusion coefficient of gases increasedshowed an increase in