The synthesis and the product of polyaniline / montmorillonite nanocomposite materials were studied. The montmorillonite was modified by quaternary ammonium salt after the exchange of its interlayer cations with Na~ . With the affinity between the quaternary_ammonium_cation/montmorillonite and the organic compounds, the aniline monomer as the guest substance was inserted in the interlayer of montmorillonite. And then the polyaniline/montmorillonite nanocomposite materials were synthesized by using the technique of in_situ polymerization. The results of XRD, SEM, TG_DTA show that the inserting of the aniline monomer can remarkably enlarge the interlayer space of montmorillonite, and the polyaniline/montmorillonite nanocomposite materials completely lose the X_ray diffraction, morphological and thermal properties of montmorillonite and the quaternary_ammonium_cation/montmorillonite. It is also shown that montmorillonite layers have been peeled off and dispersed into polyaniline and the polyaniline/montmorillonite nanocomposite materials have been synthesized.