: This research involves the investigation of the preparation and characterization of polyurethane (PU), polyurethane/poly (ether block amid) (PU/PEBA) blend membranes and also polyurethane/poly (ether block amid)/silica nanocomposite membranes for gas separation. polyurethane was synthesized by two step polymerization from Ingredients of isophorone diisocyanate (IPDI), polytetramethylene glycol (PTMG), 1, 4-butanediamine (BDA) and 1, 4-butanediol (BDO) in mole ratios of 3:1:2. A series of poly (ether block amid) (PEBA) MV3000 and MH1657 have been chosen to prepare PU/PEBA blend membranes. The effects of blend composition and weight percent of silica nanoparticles on the permeability of N 2 , O 2 , CO 2 and CH 4 were investigated. All membranes were prepared by thermal phase inversion method. Physical properties of synthesized polyurethane and prepared blends were investigated by FT-IR; XRD and SEM. Fourier transform infrared (FTIR) spectroscopy was used to verify the chemical structure of the resulting polymer and blend membranes.Results of FT-IR indicated that with increase the amount of PEBA in blend membranes, phase separation of hard and soft segments decreased, it is due to the more population of OH groups of poly(ether block amid) and N-H groups in hard segments of polyurethane.XRD results showed that with increase the amount of PEBA in blends, intensity of picks increase that it’s a usual phenomenon.The SEM micrographs of prepared samples shown that PU/PEBA blend membranes were compatible immiscible systems, that miscibility of blends increase with increase by wt% of PEBA.The results of gas permeation experiments represented that by increasing the amount of PEBA in PU/PEBA blend membranes, permeability of the all gases decreased and selectivity increased. The results of gas permeation experiments represented that by increasing PEBA in PU/PEBA blend membranes, permeability of the pure gases was decreased while selectivity was increased. The gas separation performance of the prepared membranes has been comprised to Robeson upper bound. The prepared blend membranes have good performance in separation of CO 2 .To investigate the effect of silica nanoparticles on permeability properties of blend membranes,Nanoparticles of silica made by method of sol – gel .Nanocomposite membranes polyurethane / poly (ether block amid) / silica containing over 40: 60 of polyurethane: poly (ether block amid) were prepared and evaluated. The prepared polyurethane/poly (ether block amid)/silica membranes were characterized using FTIR, XRD and SEM. The obtained SEM micrographs and FT-IR spectra’s confirmed that the silica nanoparticles distributed in polymer matrix homogenously. From XRD patterns of nanocompoites membranes excluded that crystallinity of prepared samples decreased with increase the weight percent of nanoparticles on them. Gas permeation properties of nanocomposite membranes with silica content of 2.5, 5, 10 and 20 wt% was studied for pure gases at 10 bar pressure and at 30°C. The obtained results indicated the decrease in permeability of all gases, but the increment in CO 2 /N 2 , CO 2 /CH 4 and O 2 /N 2 selectivities by increasing the content of silica nanoparticles in nanocomposite membranes. Keywords: Membrane; Gas separation; Polyurethane; Poly (ether block amid); Blend; Silica; Nanocomposite