A carbon fiber-reinforced silicon carbide (Cf/SiC) composite was fabricated with ZrB2 via the liquid silicon infiltration (LSI) method. A prepreg was prepared by impregnating the phenolic resin with the ZrB2 powder. The as-LSIed composites were tested for 5 min with an oxyacetylene torch to evaluate their ablation and oxidation properties under an ultra-high temperature environment. The ZrB2 powders and SiC matrix between carbon fiber bundles generated a dense ZrO2-SiO2 layer, which inhibited further oxygen diffusion into the composite and minimized the ablation and oxidation of the carbon fibers. Weight loss and linear ablation rate were further reduced with the addition of ZrB2 to the Cf/SiC composite; moreover, the synergistic effect of ZrB2 and SiC reinforced the ablation properties with increased ZrB2 content. ZrB2 also reduced the amount of residual silicon, which was detrimental to the mechanical properties of Cf/SiC composite.