A series of interpenetrating polymer networks of polyurethane-polystyrene were synthesized under high pressure. The morphology, dynamic mechanical properties, and density behavior were analyzed to determine the effect of the synthesis pressure on the degree of interpenetration of the component polymers. The composition of the polyurethane and polystyrene was fixed at 50% /50% by weight. The dynamic mechanical behavior showed changes form the heterophase structure to the homogeneous single-phase IPN structure when the synthesis pressure was increased. IPN’s with a theoretical molecular weight between cross-links(Mc) of 2000 synthesized above 10000kg/cm2 showed on sharp glass transition, intermediate between the two component Tg’s, and they were optically transparent. The morphology agreed well with the dynamic mechanical behavior, with dispersed domain sizes in the range of tens of angstroms. These results indicated that the IPN’s formed were the completely interpenetrated networks of the polyurethane and polystyrene.