A series of nature clay montmorillonite (MMT) filled castor oil-based polyurethane (PU)/epoxy resin (EP) graft interpenetrating polymer network (IPN) composites were prepared. The damping properties, thermal stability as well as mechanical strength of the IPN composites were studied systematically in terms of the composition. Results reveal that the addition of MMT can improve the damping properties of PU/EP IPN and can heighten the thermal decomposition temperature. Mechanical tests show that the tensile strength and impact strength of the IPN composites are significantly improved by the incorporating of MMT, especially when the MMT contents are 1% and 3%. It is expected that the MMT modified IPN composites may be used as structural damping materials in future. (C) 2011 Elsevier B.V. All rights reserved.