Polyacrylonitrile-metal-O-montmorillonite nanocomposites were prepared by means of in situ polymerization. The Na-montmorillonite was modified by quaternary ammonium salt after the exchange of its interlayer cations with Ni2+. Infrared spectra and X-ray powder diffraction were employed to characterize the obtained polyacrylonitrile-metal-O-montmorillonite. The results of FTIR show that metal-O-montmorillonite has been concerned with the polymerization of PAN. The result of X-ray powder diffraction shows that the inserting of the acrylonitrile monomer can enlarge the interlayer space of montmorillonite. It is also shown that the polyacrylonitrile-metal-O- montmorillonite nanocomposites have been synthesized. The thermal stability of PAN /MMT is better than that of pure PAN.