Abstract: A number of polyurethane based on liquid,chloroprene-hydroxyethyl methacrylate copolymer (CP-HEMA)-polystyrene (ST) interpenetrating polymer networks (PU-ST-IPN) were synthesized. Some physical properties were examined and density behavior was investigated.For 60% polyurethane (PU) system, the tensile strength and density were found to be increased clearly. Transmission electron micrographs showed the existence of phase separation and the sizes of ST domains dispersed in PU phase were esfimated about 500-4000Å. In comparison with correlative PU-MMA-IPN system, the IPN prepared had a more extensive phase separation and boundaries because of the higher compatibility in the PU-MMA-IPN.No phase inversion has been observed till up to 60% content of PU in system at which ST phase remained as a dispersed phase and PU as a continuous phase.It was considered as a result of the greater rate of PU forma-tion than that of polymerization of ST. The relatively uniform distribution of domain size and the density effect indicated that the interentanglements between the two net-works existed in the system, to some extent, would cause the incompatible system to become a partially compatible one.