Abstract: Organoclay modified polystyrene composites were prepared by melt-compounding polystyrene (PS) resin and organic montmorillonite (OMMT). The structures of the OMMT/PS composites were studied by XRD and high resolution TEM (HRTEM). The pyrolysis behavior and fire performance of the composites were investigated by means of TGA, high temperature degradation, horizontal burning, oxygen index and cone calorimetry experiments. The results indicate that the OMMT/PS composite is an intercalated composite. Compared with pure PS, adding a small amount of organoclay can evidently improve the flame retardancy of the OMMT/PS composite. The OMMT/PS composite with OMMT/PS mass ratio of 2/100 exhibits much higher thermal stability, of which the degradation temperature with 10% and 50% mass losses can increase by 7.8 ℃ and 15.2 ℃, respectively. The residue percentage at 500 ℃ and oxygen index rise by 2.6% and 1.4%, respectively, while the peak heat release rate decreases by 21.0%. The shielding of OMMT layers to the intercalated polymer chains and the formation of a continuous and compact char residue layer, which covers on the burnt polymer composite surface, can prevent the polymer from further degradation and decrease its degradation rate. The increase of melt viscosity of the OMMT/PS composite decreases the transfer rate of the combustible small-molecular compounds to the flame zone and thus suppresses the supply of fuel to the fire. The flame retardancy of the composites is enhanced due to both factors.