and soft segments decreased and chain mobility has been restricted, resulting in the increase in glass-transition temperature of the soft segment, by content of the PEG in polyurethane structure. It is due to the more population of ether group in PEG in comparison to PTMG to connect to N-H groups in hard segments via hydrogen bonding. The permeability of N 2 , O 2 , CH 4 and CO 2 were determined using constant pressure method at the different pressure and temperature. The results of gas permeation experiments represented that by increasing the ether group content in polymer structure, permeability of the pure gases decreased and selectivity increased. The glassy behaviour was observed with increasing pressure and the content of ether groups in polymer structure. For example at PU100 (containing 100 %wt of PEG in soft segment), by increasing pressure, permeability of pure gases decrease but the selectivities increase. The results indicated that the permeability of CO 2 and CH 4 in PU0 (containing 100 %wt of PTMG in soft segment) is 132.52 and 16.24 barrer that decrease to 20.26 and 0.64 barrer in PU100, respectively. Polyurethane-silica nanocomposite membranes were prepared by thermal phase inversion method. The prepared polyurethane-silica membranes were characterized by using FT-IR and SEM. The obtained SEM micrographs and FT-IR spectras confirmed that the silica nanoparticles distributed in polymer matrix homogenously. Gas permeation properties of polyurethane-silica nanocomposite membranes with silica content of 5, 10 and 30 %wt was studied for pure gases at 10 bar pressure and at 25°C. The obtained results showed the reduction in permeability of all gases in PU0 and PU75 (containing PEG/PTMG ratio of 3 in soft segment) based nanocomposite membranes, 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 them. In PU100 based nanocomposite membrane, permeability of all gases increased but the selectivity decreased. The reduction of selectivity is due to increasing nano gaps around the silica nanoparticles. On the other hand, pure polymer was saturated by ether groups that even the presence of ether groups of the silica nanoparticles no effect on the increasing selectivity and thus the selectivity decreases. The permeability of condensable gases in comparison to non-condensable ones more increase by increasing the silica content in polymer structure. Hence, the selectivity of condensable gases in polyurethane considerably decreased. In PU100 based nanocomposite membrane, permeability of CO 2 and N 2 was increased from 20.26 and 0.22 barrer to 50.10 and 0.72 barrer, respectively, by increasing of the silica nanoparticles from 0 to 30 wt%.