Sodium-montmorillonite (Na+ -MMT) was organically modified by cetyltrimethyl-ammonium bromide．Using the organically modified montmorillonite (OMMT)

a high impact polystyrene (HIPS)／OMMT composite was prepared by a melt intercalation method．X-ray diffraction patterns revealed that the spacing distances between the layers of the MMT were 1.51 nm for Na+ -MMT

2.18 nm for OMMT and 3.44 nm for HIPS/OMMT(at 5 phr) composite respectively while for the HIPS／Na+ -MMT (at 5 phr) composite the spacing was 1.48 nm

showing that the HIPS chains were intercalated into OMMT by the melt intercalation process．The cone calorimeter was used to assess the flammability of the HIPS／OMMT and the HIPS／Na+ -MMT composites．The results demonstrated that comparing to the pure HIPS the heat release rates (HRR)

the mass loss rates (MLR) and the smoke production rates (SPR) for the HIPS／OMMT composites are all reduced dramatically

e. g. peak HRR of the HIPS／OMMT composite with only 5 phr OMMT decreases by 44.8％．In contrast

the properties of HIPS／Na+ -MMT composites changed only slightly except in cases of high loadings．Meanwhile

the HRR of the mixture of HIPS and cetyltrimethyl-ammonium bromide

and the HRR of the mixture of HIPS

cetyltrimethyl-ammonium bromide and Na+ -MMT were also measured and compared. It was found that the ammonium salt alone did not reduce flammability of HIPS significantly．The flame retardant mechanism of the HIPS／OMMT composite was discussed based on the analyses of the cone calorimeter data and the burnt residues. It was concluded that the resulted flame retardancy of the composite was mainly due to the char layer which acted as both a barrier to mass transport of the degraded products and a thermal barrier to the feedback of the combustion heat．The study shows that the HIPS／OMMT composite is a promising environmental friendly flame retardant material.