Crosslinked membranes were prepared by the addition of a crosslinking agent of benzoyl peroxide (BPO) to the hydroxyl-terminated polybutadiene (HTPB) and 4,4’-dicyclohexylmethane diisocyanate (H(12)MDI)-based polyurethane (PU) solutions. The stress-strain properties of segmented HTP-B based PUs were changed by the crosslinking between soft-soft segments. The gas permeabilities of HTPB-based PU membranes with a suitable amount of crosslinking agent were higher than those of uncrosslinked membranes, which is due to the stretched and extended soft-segment molecular chains. The reproducibility of gas permeability was improved by the crosslinked membranes. Thermal stability conducted by TGA was increased with increasing crosslinking agent content. FTIR was utilized to identify the segregation between hard and soft segments and structure change, which affects the gas-transport properties. The change of glass transition temperature was detected by DSC, which can be used to manifest the degree of crosslinking of these membranes. The results of TGA, FTIR, and DSC measurements explain the crosslinking degree with different BPO content and, hence, the gas permeabilities as well.