Acrylate-modified polyurethane (PU) resin was used and then interpenetrated with unsaturated polyester resin (UPR), to form interpenetrating polymer networks (IPNs) and gradient IPNs cured at room temperature. The polymerization processes were traced through IR techniques, through which the phase separation in systems could be controlled effectively. Results of the morphology and miscibility of the multi-phase materials, obtained by TEM, indicate that the domains in these systems were constricted to the nanometer range. Thermomechanical analyzer (TMA) detection results showed that through interpenetration between networks; the glass transitional temperature ranges of systems can be linked up apparently. Mechanical characteristics showed that the IPNs obtained exhibit different types of characteristics varying from elastomer to brittle plastics. The systems were composited with BaTiO3 fibers and the composite techniques were determined. Also, the morphology and mechanical properties were discussed in detail. (C) 2003 Elsevier B.V. All rights reserved.