The effects of elastomer type on morphology, flammability and rheological properties of high-impact polystyrene/Mg(OH)(2) based on encapsulated by polystyrene have been investigated. The ternary composites characterized by cone calorimetry, horizontal burning rate, limiting oxygen index (LOI), rheology and SEM. Morphology was controlled using poly[styrene-b-(ethylene-co-butylene)-b-styrene] triblock copolymer (SEBS) or the corresponding maleinated SEBS (SEBS-g-MA). As revealed by SEM observations, composites of HIPS/SEBS/Mg(OH)(2) exhibit separation of the filler and elastomer and good adhesion between SEBS and the filler, whereas composites of HIPS/SEBS-g-MA/Mg(OH)(2) exhibit encapsulation of the filler by SEBS-g-MA. The flame retardant and rheological properties of ternary composites were strongly dependent on microstructure. The rheological test showed that the composites with encapsulation structure exhibit a stronger solid-like response at low frequency than those of the composites with separate dispersion structure. The combustion tests showed that the composites with encapsulation structure showed higher flame retardant properties than those of separate dispersion structure at optimum use level of SEBS-g-MA. However, with the increase of the content of SEBS-g-MA, the flame retardancy of the composite declined somewhat which can be explained that the SEBS-g-MA coating acts as a heat and mass transfer barrier due to the formation of encapsulation structure. (c) 2007 Wiley Periodicals, Inc.