Abstract: The high silica fiber/ceramicizable phenolic resin composites were prepared by using AlB₂ and SiC particles filled phenolic resin as matrix and high silica fiber as reinforcement. The effects of different amounts of AlB₂ particles on the performance of high silica fiber/ceramicizable phenolic resin composites were studied at room temperature and after 1200℃ pyrolysis, respectively. The enhancement mechanism of AlB₂ particles on pyrolysis products of the high silica fiber/ceramicizable phenolic resin composites was analyzed. The results reveal that as the amount of AlB₂ particles increasing, the flexural strength of the high silica fiber/ceramicizable phenolic resin composites gradually decreases at room temperature, while the flexural strength of the 1200℃ pyrolytic composites displays a tendency to increase first and then decrease at high content of AlB₂. When the mass ratio of AlB₂ particles to phenolic resin is 12%, the flexural strength of the pyrolysis products is improved most significantly, 16.4% higher than that of the composites without AlB₂ particles. AlB₂ particles react in an aerobic environment at 1200℃ to form a co-melt composed of B₂O₃, Al₂O₃ and Al₂₀B₄O₃₆, which fills the pores of the pyrolysis products of the phenolic resin, significantly reducing structural defects of the pyrolysis products, preventing further oxidation of the internal materials. Therefore, the mechanical properties of the pyrolysis products are improved.