Polyurethane (PU) membranes with high permeability and selectivity properties, good film formation ability in addition to significant mechanical properties are promising materials for gas separation industry. Relatively low carbon dioxide concentration of incoming feed, as result of low flue gas pressure, is one of the limitations in application of polymers membranes as CO 2 gas separation membranes. Hence achievement of higher selective membrane is necessary. In present research, in order to increase the tendency of PU surface membranes to adsorb CO 2 in comparison to H 2 and N 2 gases, surface modification of polyurethane membrane by CO 2 adsorbent agents was studied. The chemical modification was carried out in two steps, the first step was diisocyanate modification and the second step, modification with amines. The second step conditions were optimized by ATR and contact angle tests. For this purpose, three amines with different number of amine groups and chain length, in addition to water were applied. The effect of toluene as solvent was also determined separately. ATR, contact angle and grafting yield results indicated high efficiency surface modification. For example, contact angle of Triethylenetetramine modified sample decreased about 70 degrees. In amine modified samples, grafted molecule volume, amount of polymer swelling in the presence of each amine, grafting efficiency, relative concentration of amine groups, level of phase separation and crystallinity of each sample, effected on the permeability and selectivity of different gases. Additionally, the type and amount of interaction between each gas and modified surface was also affecting on the gas separation properties of the modified membrane. In comparison to other gases, CO 2 has significant condensability and polarizability, therefore the CO 2 solubility coefficient of amine modified samples increased with increasing the concentration of amine groups in different samples; As far as, solubility of CO 2 in PU-TETA sample increased 1.82 times. On the other hand, the diffusivity coefficient of different gases was reduced due to presence of new molecules on the surface and steric hindrance created by the crystallinity of modified surface layer. The membrane swelling in the surface modification process also affected the penetration coefficient of amine-modified specmens. Diethylenetriamine modified sample swelled more, due to the closeness of its solubility parameter with the polyurethane membrane. Therefore, its diffusion coefficient was higher than the other two modified samples and had higher permeabilities for different gases. Generally, ideal selectivity of all modified samples increased from 15 to 75 percent with the exception of water modified one. Due to presence of water molecule between the polymer chains and formation of hydronium carbamate strong bonding, the disorption process of CO 2 from water modified surface became harder and the ideal selectivity of water modified sample decreased. CO 2 /H 2 mixed selectivity of PU-W sample 1.3 times increased. One of the factors affecting the selectivity of mixed gas was accelerating of desorption phenomena of CO 2 by H 2 and N 2 gases. This effect was more explicit for water modified sample, due to difficulty of CO 2 disorption process from hydronium carbamate group. Plasticization of the surface layer, because of competitive adsorption of CO 2 , was another effective factor. The plasticization of the surface layer, especially for CO 2 /H 2 mixed gas test due to DCO 2 /DH 2 and SCO 2 /SH 2 increasement, resulted in improvement of selectivity of mixed gas in comparison to pure gas. Keywords : Membrane, Polyurethane, Permeability, Selectivity, Chemical surface modification.