Abstract: Expandable graphite (EG) and magnesium hydroxide (MH) were used to prepare halogen-free flame-retardant polypropylene(PP) composites (EG/PP, MH/PP and MH/EG/PP) through melt blending. The thermal degradation process was investigated with the thermogravimetric analysis (TGA). The flame retardant properties were evaluated in terms of limiting oxygen index (LOI) and vertical testing UL-94. The combustion behavior was studied with cone calorimeter. As a result, the significant synergistic flame retardant effect between MH and EG was revealed. When the mass fraction of flame retardant was 60%, the LOI value of the composites with MH/EG at a mass ratio of 5 : 1 reached 29.7, which showed an increase of 11.2% in comparison with the composites with MH. MH/EG exerted a synergistic effect in reducing heat release rate (HRR). The peak-HRR of MH/EG/PP composites was decreased by 73.9% and 34.2% respectively in comparison with that of PP and MH/PP. The thermal weight loss rate of the composites was greatly depressed when EG and MH were applied together in PP. Combined with the results of the residue morphology, the flame retardant mechanism of the enhanced effects of char layer in the barrier effects of heat and O₂ transfer was revealed.