The present study focuses on the thermal response of carbon ?ber-reinforced phenolic composites, where the matrix has been modi?ed with different reinforcements. Two types of materials, multiwalled carbon nanotubes and zirconium diboride(ZrB_2), were used in a new design of mixture to produce the heatresistant ablative composite system. The CNT/ZrB_2/carbon/phenolic nanocomposite(Z/NT-CR) system corresponding to CNT/carbon/phenolic nanocomposite(NT-CR) showed a reasonable decrease in mass loss and the ablation rate as compared to carbon/phenolic composite(CR). However, substantial drop in two factors was found for Z/NT-CR as compared to carbon/phenolic and NT-CR. Ablation mechanisms for all three composites were investigated by thermal gravimetric analysis in conjunction with microstructural studies using a ?eld emission scanning electron microscope. The microstructural studies revealed that CNTs acted as an ablation resistant phase for protection against 2000 °C, and the conversion from ZrB_2 to Zr O2 played an important role as an insulator in the performance of char layer in the ablation resistance. The present study focuses on the thermal response of carbon? Ber-reinforced phenolic composites, where the matrix has been modi? Ed with different reinforcements. Two types of materials, multiwalled carbon nanotubes and zirconium diboride (ZrB_2), were used in a new design of mixture to produce the heat resistant ablative composite system. The CNT / ZrB 2 / carbon / phenolic nanocomposite (Z / NT-CR) system corresponding to CNT / carbon / phenolic nanocomposite (NT-CR) showed a reasonable decrease in mass loss and the ablation However, substantial drop in two factors was found for Z / NT-CR as compared to carbon / phenolic and NT-CR. Ablation mechanisms for all three composites were investigated by thermal gravimetric analysis in conjunction with microstructural studies using a? eld emission scanning electron microscope. The microstructural studies revealed that CNTs acted as an ablation resistant phase for protection against 2000 ° C, and the conversion f rom ZrB_2 to Zr O2 played an important role as an insulator in the performance of char layer in the ablation resistance.