The nano.intercalating technique and the simultaneous interpenetrating polymer network(IPN)method were used together to prepare the nanocomposite of Polyurethane(PU)/Poly(Methyl Methacrylate)(PMMA)/Organo.Montmorillonite(OMMT).The structures,morphologies and mechanical properties of the prepared nanocom-posite and the corresponding PU/PMMA-IPN system were then investigated by means of XRD,SEM ,TEM and me-chanical testing.The results show that a part of montmorillonites well disperse in the polymer matrix in the form of sheets(8 to 20 nm in thickness and 50 to 200 nm in length),forming an intercalated/exfoliated nanocomposite,that the mechanical properties of PU/PMMA/OMMT nanocomposite are obviously superior to the coresponding PU/PMMA.IPN system.and that there exist hydrogen bonds between the OMMT and PU or/and PMMA in the nano-composite,thus enhancing compatibility and the interactions among OMMT,PU and PMMA,and considerably im-proving the mechanical properties of the nanocomposite.The effects of preparation conditions,such as the mass ra-ti0 of PU to PMMA,the contents of OMMT,benzoyl peroxide(BPO)and ethyleneylycol dimethacrylate(EGDMA),and the 1,4-butylene diol(BDO)coeficient,on the mechanical properties of the nanocomposite were also investi-gated,concluding that when the mass ratio of PU to PMMA is 60/40,the contents of OMMT,BPO and EGDMA are respectively 5.0%,0.80%and 2.0%,and the BDO coefficient is 0.80，the nanocomposite can be prepared in
optimal conditions.