Abstract: Mesoporous molecular sieve(MCM-41) and Cr₂O₃ were used on intumescent flame retardant (IFR) to flame retardant natural rubber(NR). In order to study the flame retardant synergistic effects of MCM-41 and Cr₂O₃, two synergists of different components and IFR were used to flame retardant natural rubber(NR). The oxygen index test, thermogravimetric analysis, cone calorimetric analysis, tensile test and the analysis of the residual carbon scanning were investigated for the flame retardant system. The results show that when only expansion flame retardant system is added to the natural rubber, the mechanical properties of the flame retardant system are dropped dramatically and the thermal performance almost have no improvement. While tensile strength and elongation at break of the composites are improved with the increase of Cr₂O₃ and MCM-41. The IFR-MCM-41-Cr₂O₃ composite flame retardant effect achieves the best when IFR content is 36% (mass ratio to the natural rubber), MCM-41 content is 1%, Cr₂O₃ content is 3%. And the peak value of heat release rate and total heat release of burning decrease significantly. The IFR-MCM-41-Cr₂O₃/NR composites could form compact charred layer after burning. The limiting oxygen index (LOI) can reach 26.5% and the vertical combustion (UL-94) is V-0 level.