For the purpose of oxygen enrichment from air, the gas permeability and selectivity of an ionic polyurethane membrane was under investigation. Membranes of ionic polyurethane were prepared by step-growth polymerization of hydroxyl terminated polybutadiene (HTPB) and 4,4’-dicyclohexylmethane diisocyanate (H(12)MDI). The ionic group was introduced by adding N-methyldiethanolamine (MDEA) as the chain extender of which the tertiary amines were complexed with cupric ions. The effect of hard segment content, polymerization method, peroxide introduction, and the amount of cupric ion on gas permeability were investigated. It was found that the binding of hard segment and the flexibility of soft segments had subtle effects on gas permeability. Membranes of the same composition were synthesized through two different procedures, one- and two-stage polymerization. The former contains large hard segment of cluster aggregation and flexible soft segments had a higher gas permeation rate. When a crosslinker, benzoyl peroxide, was added, the crosslinkage within soft segments hindered cluster formation by hard segment aggregation, the permeability increased. Furthermore, CuCl2 addition enhanced hard segment aggregation, more hard segments formed cluster aggregates and less dispersed in soft segment region, which also increased permeability. However, excess CuCl2 addition resulted in CuCl2 piling up in the soft segment region, which restricted the movement of soft segments and therefore reduced the gas permeability.