ZrB₂ modified carbon–phenolic (C–Ph) composites are fabricated by the impregnation method, and the ablation performance of the composites is tested using an oxyacetylene flame. In addition, the microstructure of the ablation zone is observed with a scanning electron microscope (SEM). The results indicate that the linear ablation rate decreases with increasing ZrB₂ content, and the ZrB₂–C–Ph composites have their lowest linear ablation rate when the ZrB₂ to phenolic resin weight ratio is 0.09 : 1. The main ablation mechanisms of the composites with low loading of ZrB₂ are oxidation and char spallation. When more ZrB₂ particles are introduced, a continuous protective ZrO₂ layer is properly formed on the ablated surface of the specimen, and the stripping of the protective ZrO₂ layer and the oxidation of carbon are the main ablation mechanisms. Furthermore, the forming–stripping process of a protective ZrO₂ layer occurred continuously and was repeated during the ablation process.