: This research involves the investigation of the preparation and characterization of polyurethane (PU), polyurethane/poly (vinyl alcohol) (PU/PVA) blend membranes and also polyurethane/poly (vinyl alcohol)/silica nanocomposite membranes for gas separation. polyurethane was synthesized by two step polymerization in mole ratios of 3:2:1 from isophorone diisocyanate (IPDI), 1, 4-butanediamine (BDA) and polytetramethylene glycol (PTMG). A series of poly (vinyl alcohol) (PVA) with 15, 30, 60, 145 and 200 kilodalton molecular weights have been chosen to prepare PU/PVA blend membranes. The effects of molecular weight of poly (vinyl alcohol), 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. In first step for investigation the effect of PVA molecular weight on gas separation properties of PU/PVA blend membranes, all blends were prepared in 10 wt% of PVA in membranes. 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 obtained by FT-IR spectrum of the prepared blends indicate that phase separation of hard and soft segments in polyurethane was decreased by increasing the molecular weight of PVA in blends, it is due to the more population of OH groups to connect to N-H groups in hard segments via hydrogen bonding. Also the disappearance of stretching vibration bound at 1141cm ?1 indicate that the crystalline portion of poly (vinyl alcohol) chains that attribute the intramolecular hydrogen bonding is formed between two neighboring OH groups, were removed by blending with polyurethane. In order to explore this behavior, XRD experiments were performed to investigate crystallinity of the blend polymers. Obtained results from XRD patterns shown that crystallinity of prepared blends have been decreased with increase of PVA molecular weight in PU-based blend membranes. As in PU/PVA blend with lowest PVA molecular weight no any change was observed in XRD patterns for pure PVA and blended with PU, that indicate the crystalline chains of PVA distribute in polyol soft segments of PU. Whereas for PU/PVA blend with highest PVA molecular weight, decreasing the number of picks from two sharp picks in pure PVA to one broad pick in PU/PVA blend indicate that PVA chains with any crystalline portion entangled with PU chains. SEM micrographs of prepared samples shown that PU/PVA blend membranes were compatible immiscible systems in low molecular weights of PVA, that miscibility of blends increase with increase molecular weight of PVA. It is due to the more incorporation and entanglement of PVA chains with PU chains. The results of gas permeation experiments represented that by increasing the molecular weight of PVA in PU/PVA blend membranes, permeability of the pure gases was decreased while selectivity was increased. The reduction in the permeability of polar CO 2 gas in comparison to non-polar ones by increasing the available OH group’s content in membrane structure with increase the molecular weight of PVA in blends was lower. Hence, the selectivity of CO 2 gas over N 2 and CH 4 in blend membranes increased. The gas separation performance of the prepared membranes has been comprised to Robeson upper bound. The