Flame retarding high impact polystyrene (HIPS) with a reduced amount of halogen flame retardant was prepared by using phosphoric and inorganic flame retardants to improve the shortcomings of the halogen-containing counterparts, such as emission of poisonous gas and environmental pollution. Red phosphorous and metallic flame-retardants were applied for this purpose. Effects of these non-halogen type flame retardants on the thermal and mechanical properties, as well as flame retardancy, of the flame retarding HIPS were examined by thermogravimetric analysis, differential scanning calorimetry, Izod impact strength, and the UL94 flame retardation test.