Poly(urethane-imide) (PUI) membranes are prepared by blending polyurethane (PU) prepolymer with different ratio of poly(amide acid) (PAA). PU prepolymer are synthesized by two-stages process with hydroxyl-terminated polybutadiene (HTPB), 4,4´-dicyclohexylmethane diisocyanate (H12MDI) and 1,4-butane diol (1,4-BD). While 4,4´-diaminodiphenyl ether (ODA) and 3,3´–4,4´-benzophenonetetracarboxylic dianhydride (BTDA) are reacted to forming the polyimide (PI) oligomer solution of poly(amide acid) (PAA). IPN structures are formed through reaction between PU prepolymer and PAA and unsaturated CC double bond between different HTPB segment of PU. All PUI membranes show three steps of weight loss curves and PI shows one step weight loss curve by thermogravimetric analysis (TGA). Compositions lower than 50 wt.% of PU content (0.5-PUI) possess better compatibility between PU and PI and the glass transition temperature of PU hard segment (Tgh) of PUI membranes disappeared. The Tgh values, maximum degradation temperature and char yield of PUI membranes are increased as the increase of PI content. The gas permselectivities of 0.5-PUI are 12 times than that of pure PI.