Abstract: MgO and (or) microencapsulated red phosphorus (MRP) were incorporated into high impact polystyrene (HIPS) matrix by melt-compounding and a series of MgO-MRP/HIPS composites were prepared. The flame retardancy of MgO-MRP/HIPS composites was investigated by limiting oxygen index (LOI), vertical burning test (UL-94), cone calorimetry, TGA, SEM, XRD and FTIR. The results indicate that either MgO or MRP has slight flame retardant effect on HIPS when they are used alone, but there is obvious synergism between MgO and MRP on the flame retardancy of HIPS when they are combined at a proper mass ratio. When the mass ratio of MgO:MRP:HIPS is 35:15:100, the LOI of the MgO-MRP/HIPS composite is 24.7% and the UL-94 vertical burning rating of the composite can reach V-0. Both heat release rate and total heat release of composite are decreased noticeably. The composite with this composition exhibits excellent flame retardancy. There is no interaction among HIPS, MgO and MRP at high temperature in anaerobic condition. However, both MgO and MRP can promote charring when the polymer is degraded or burned in air. A continuous and compact charred residue layer can be produced on the surface of the MgO-MRP/HIPS composite, which acts as a barrier against fire and gives rise to increased flame retardancy. The burnt residue of the MgO-MRP/HIPS composite consists of crystalline MgO and phosphorus-containing amorphous black char. In addition, MRP also plays a flame retardant role in the gas phase during combustion of the composite.