Abstract: C/C composites with densities around 1.3 g/cm³ were infiltrated with Al and Si powder mixtures to densities of 1.9-2.2 g/cm³ using reactive melt infiltration to improve their anti-ablation properties. The effects of the Si/Al mass ratio, infiltration temperature, time and atmosphere on the microstructures and ablation properties of the modified composites were investigated by XRD, SEM equipped with EDS, electron-probe micro analysis and oxygen-acetylene ablation tests. Results indicate that homogeneous Al+SiC and Al₄SiC₄+SiC were respectively formed in the C/C composites after infiltration at 1 500℃ for 2 h and 1 600℃ for 6 h in argon. A gradient of SiC+Al was formed after treatment at 1 600℃ for 6 h under vacuum with the Al content decreasing from the inside to the outside of the composite. The gradient C/C modified by SiC+Al with a Si/Al ratio of 3 has the best ablation resistance of the three, with a mass ablation rate of -1.0×10^-3 g/s in an ablation test at 2 500℃ for 60 s.