The magnesium hydroxide (MH) whiskers were modified with 3-(trimethoxysilyl) propyl methacrylate (γ-MPS) followed by in situ copolymerization of styrene. The high impact polystyrene (HIPS)/MH composites were prepared by melt compounding of HIPS with untreated MH whiskers and modified MH whiskers (MMH), respectively. Their morphological structure and rheological properties were characterized by scanning electron microscopy and capillary rheometer. The results show that the HIPS/MH composites are a shear-thinning non-Newtonian, its apparent viscosities are bigger than those of the HIPS melt. With increasing MH loading, apparent viscosity of HIPS/MH composites increase. The PS encapsulated on MH whiskers surface enhanced the interface adhesion between inorganic particles and HIPS matrix and improved the flow abilities of the HIPS/MH composites melts.