Abstract: The conventional thermosetting phenolic resin exhibits significant brittleness. This paper utilized hexamethylenetetramine (HMTA) to chemically toughen phenolic resins. And the comprehensive effect of carbon nanotube synergistic modification on improving the heat-resistant properties of the phenolic resin was also examined. The results indicate that the mechanical properties of the HMTA-modified phenolic resin present a tendency of first increasing and then decreasing with the increase of the HMTA content. The highest mechanical properties are observed in the modified phenolic resin with 2.5wt%HMTA, as its compression strength reaches up to 393 MPa and the bending strength up to 149 MPa, representing increases of 24% and 62%, respectively, compared to the unmodified phenolic resin matrix. The fracture morphology reveals that the low content of HMTA creates numerous "sea-island" structures within the modified phenolic matrix, which significantly contributes to the improvement of the mechanical properties of the phenolic matrix. However, HMTA can lead to an obvious decrease in the heat-resistance of phenolic resins. Adding carbon nanotubes to the 2.5wt%HMTA toughened phenolic system can increase the T_d5 (Pyrolysis temperature at 5wt% mass loss) of the resin system to the level of the unmodified phenolic resin, while also exhibiting excellent toughened mechanical properties. This lays an important foundation for the design and development of new structural/thermal protection integrated phenolic resin matrices and their composites.