Abstract: The synergistic flame retardant system consisting of cage pentaerythritol phosphate (PEPA) and expandable graphite (EG) was added to the natural rubber (NR) to prepare EG-PEPA/NR composites. The effects of PEPA and EG with different mass ratios on the flame retardancy and mechanical properties of EG-PEPA/NR composites were investigated by limiting oxygen index(LOI) and vertical combustion(UL-94) test, thermogravimetric analysis, cone calorimetry, tensile test and carbon residue SEM and FTIR. The results show that the flame retardancy and mechanical properties of EG-PEPA/NR composites perform the best when PEPA to EG with the mass ratio of 33:7 are added to NR. Its LOI reaches 28.1% and the UL-94 reaches V-0 level. The amount of carbon residue increases from 27.5% to 33.6% at 600℃. The peaks of total heat release and heat release rate are 96.2 MJ·m^-2 and 512.4 kW·m^-2, respectively, decreasing by 22.2% and 40.3% compared to the pure NR sample. The tensile strength and elongation of EG-PEPA/NR at break also reach 13.8 MPa and 368%, respectively.