Ethylene and propylene are two of the most important petrochemicals which their main sources are steam cracking and FCC processes. Currently, separation of olefin and paraffin gases is carried out using low temperature distillation process, which is very expensive and energy consuming.Due to the high costs and complexity of cryogenic distillation, using polymeric membrane separation processes is gradually growing in popularity for olefin/paraffin separation purposes. It has been found that polymeric membranes, e.g., polysulfone , cellulose acetate, and PDMS are not suitable for this kind of separation because of theirvery small separationfactors. On the other hand, much better olefin/paraffin separation characteristics were achieved by using polyimides as membrane materials. Undesirably, one problem identified with polyimide films for propylene/propane separations was plasticization effect.In this research, the separation of ethylene/ethane and propylene/propane by polyimide-silica nano-composite membranes was studied. To this aim, sol-gel method was used for preparation of polyimide nano-composite. Polyimide and polyimide-silica nano-composite membranes were prepared by thermal phase inversion method. The prepared membranes were characterized using FT-IR, SEM, and XRDanalyses. The obtained SEM micrographs and FT-IR spectras confirmed that the silica nanoparticles were homogenously distributed in polymer matrix. The performance of the prepared membranes in ethylene/ethane and propylene/propane separation was evaluated at pressure and temperature of 3bar and 310K, respectively. The obtained results showedincreasing in permeability of propylene and ethylene from 0.091 and 0.266 barrer, respectively, in pure polyimide to 0.159 and 0.411 barrer in the composite membrane containing 20%wt nanosilica particles. Increase in propylene and ethylene permeability is due to the enhanced solubility in polymer matrix. The obtained results indicated that selectivity values of 3.24 and 9 for C2H4/C2H6 and C3H6/C3H8 separation, respectively, by pure polyimide was increased to 6.13 and 17.9 for composite membrane containing 20%wt nanosilica particles .To investigate the effect of adding silica nanoparticles on plasticization phenomena, gas permeation of PI and PI-silica nanocomposite membranes was studied for propylene and propane single gases at pressure range of 1-8 bar and for ethylene and ethane single gases at pressures of 1, 3, and 5bar. Plasticization was observed for propylene and propane in pure PI at the feed pressure of 4 and 6 bar,respectively. These results indicated that in PI-nanocomposite membranes with silica contents of 5 and 10wt% plasticization resistant increased and the plasticization pressure became larger with increase in silica content of PI-nanocomposite membranes. However, with increasing the silica mass fraction plasticization resistance did not improve and plasticization pressure decreased in PI-nanocomposite membranes with silica content of 15 and 20%. In the case of propane, no plasticization was observed in PI-nanocomposite membranes for up stream pressures up to 8bar. Decreasing permeability for ethylene and ethane with increasing feed pressure indicated that no plasticization occurs in pure PI and composite membranes for up to 5bar feed pressure. Also, In this work, the effect of temperature on of separationethylene/ethane mixture was investigated. With increasing temperature, the permeability of the gases increased from 0.201 and 0.063barrer at 298K to 0.307 and 0.074 barerr at 310K,respectively. For propylene/propane mixture permeability increased from 0.115 barrer and .006barrer at 298K to 0.125 and 0.01barrer at 310K,respectively. Keywords: Olefin-paraffin separation ;Nanocomposite; Polyimide ; Silica; Gas permeatio