High impact polystyrene (HIPS) magnesium hydroxide (MH) composites were prepared by melt-blending. Two kinds of interfacial modifiers were used in this research, maleinated poly[styrene-b-(ethylene-co-butylene)-b-styrenel] (SEBS-g-MA) triblock copolymer and PS. The effects of the use levels of SEBS-g-MA on the flame retardancy of HIPS/elastomer/MH based on unmodified and PS-modified surface were investigated by TEM, FTIR, and combustion tests (horizontal burning test and cone calorimetry). The combustion results showed that comparing composites containing unmodified MH, the flame retarding properties of composites containing PS-modified MH were obviously improved. The increased performance can be explained that the PS covered on the surface of MH could further improve dispersion of the filler in matrix. Furthermore, there existed a critical thickness of interfacial boundary for optimum flame-retarding properties in both ternary composites based MH and P-S-modified MH. When the interfacial boundary relative thickness is less than 0.53, the introduction of SEBS-g-MA can improve the dispersion degree, leading the improvement of flame retardancy properties. However, with the increase of Interfacial boundary thickness, the SEBS-g-MA coating around MH acted as I heat and mass transfer barrier, leading to the reduction of flame retardancy. (c) 2008 Wiley Periodicals, Inc.