Abstract: Montmorillonite (MMT)/polystyrene (PS) composites were prepared by melt-compounding PS resin and different kinds of MMT. The structures of the composites were investigated by XRD and high resolution TEM. The pyrolytic charring behavior and fire performance of the composites were studied by means of SEM, X-ray energy dispersive spectrometry (EDX), XRD, oxygen index and cone calorimetry experiments. It is shown that the NaMMT/PS composite is a simple physical blend of PS and virgin MMT (NaMMT), while the organic montmorillonite(OMMT)/PS composite is an intercalated-partially exfoliated nanocomposite. When the mass ratio of MMT to PS is 6:100, the peak heat release rate(PHRR) and average heat release rate(AHRR) of the OMMT/PS composite are only 70.7% and 67.4% that of its NaMMT/PS counterpart, respectively. Meanwhile, the fire performance index (FPI) of the OMMT/PS composite is 1.2 times that of the NaMMT/PS composite, and the oxygen index value of the former is 1.0% higher than that of the latter. The content of carbon element in the residue surface of OMMT/PS composite is 2.5 times that of NaMMT/PS composite. A thick, compact and continuous carbon-containing silicate residue layer is formed on the surface of the OMMT/PS composite, which can effectively inhibit burning. The NaMMT/PS composite hardly has any charring ability. The residue of this composite derives from the pyrolysis of NaMMT completely and it exhibits very poor flame retardancy.