Abstract: Microencapsulated red phosphorus (MRP) and poly(phenylene oxide) (PPO) were used to improve the flame retardancy of high impact polystyrene (HIPS). A series of MRP-PPO/HIPS composites with different compositions were prepared by melt-compounding. The flame retardancy of the composites was investigated by horizontal burning, vertical burning, oxygen index, cone calorimetry and high temperature pyrolysis experiments. It is indicated that the composite containing both MRP and PPO exhibits better fire retardancy than its counterpart containing only PPO or MRP at the same flame retardant contents. When the mass ratio of MRP-PPO/HIPS is 10:20:70, the oxygen index of the composite is 23.9% and the horizontal burning rating and vertical burning rating of the composite can reach FH-1 and FV-0, respectively. The composite with this composition displays the optimum flame retardancy. Excessive contents of MRP can decrease the fire retardancy of the composite. It is believed that there is strong synergistic effect between MRP and PPO on the flame retardancy of HIPS. The coexistence of MRP and PPO with suitable mass ratio can decrease the heat release rate and combustion heat of the composite considerably, which lowers the temperature in the gaseous combustion zone and leads to increased flame retardancy in the gas phase. Meanwhile, the composite can produce a continuous and compact charred residue layer during pyrolysis and combustion. This char layer suppresses heat transfer and mass exchange and improves the flame retardant effect in the condensed phase. As a result, the flame retardancy of the composite is improved considerably.