adding inorganic nanofillers is one of the predominant concerns of the researchers. In the present work, the separation of ethylene/ethane and propylene/propane by cellulose acetate-silica nanocomposite membranes were studied. Silica nanoparticles were prepared via the hydrolysis of tetraethoxysilane (TEOS). Membranes were prepared by thermal phase inversion method. The prepared membranes were characterized using FT-IR, SEM, DSC and TGA methods. The results demonstrate a nano-scale and good distribution of silica particles in the polymer matrix. This confirms the desirable mixing of the silica in polymer and also a good compatibility between the two phases. The thermal properties of cellulose acetate membranes were also improved by adding silica particles. The performance of prepared membranes in ethylene/ethane and propylene/propane separation were measured at pressure of 2 bara and temperature of 35 ?C. The obtained results showed that the permeability of ethylene and propylene increased from 0.052 barrer and 0.046 barrer in pure cellulose acetate to 0.11 barrer and 0.098 barrer in the composite membrane containing 30%wt silica particles. The comparison of the selectivities of C 2 H 4 /C 2 H 6 and C 3 H 6 /C 3 H 8 indicates an increase in these quantities from 2.16 and 2.55 in pure polymer to 4.07 and 6.12 in composite membrane containing 30%wt silica particles. The diffusion and solubility coefficients were determined by the time lag method. The results showed an increase in solubility coefficient and decrease in diffusion coefficient of gases with increasing the silica mass fraction. For example the diffusion coefficient of ethylene decrease from 21.22*10 -10 cm 2 /s in pure cellulose acetate to 14.02 *10 -10 cm 2 /s in composite membrane containing 30%wt silica particles and it’s solubility coefficient increased from 2.21*10 -3 cm 3 (STP)/cm 3 cmHg to 7.85*10 -3 cm 3 (STP)/cm 3 cmHg. In order to investigate possible plasticizatio phenomena, the effect of feed pressure on separation performance and O 2 /N 2 selectivities befor and after exposure of membranes with propylene was studied. With increasing feed pressure, the permeability and solubility coefficients of gases decreased and the diffusion coefficient increased. Decreasing permeability with increasing feed pressure is consistent with the ‘dual mode sorption’ for glassy polymers and indicates that no plasticization occurs in pure cellulose acetate and composite membranes up to 8 bara feed pressure. Before the exposure of propylene, the pure and composite membrane containing 30%wt silica particles show O 2 /N 2 selectivity of 5.08