Abstract: The ablation behavior of a four-directional carbon/carbon (C/C) composite was examined in a lab-scale solid rocket motor under a high flux of combustion products containing a high content of particulate alumina. The composite consisted of three braided carbon fiber bundles at 120° to each other in the XY plane and a hexagonal array of carbon rods in the Z direction, all in a pitch carbon matrix. The rods consisted of a unidirectional array of the same carbon fibers in a pitch carbon matrix The composite was placed in the rocket motor with its XY plane perpendicular to the gas flow and its ablation rate, ablation behavior and microstructure were investigated. The flow field of the combustion products was simulated by solving the Reynolds-averaged Navier-Stokes equations. A deep pit was formed on the surface of the composite, the center of which coincides with the simulated particle accumulation area. The mechanical erosion was significantly increased when the particle impact velocity exceeded 96.82 m/s. The carbon rods were more susceptible to erosion than the surrounding fiber bundles. The maximum ablation rates of the carbon rod and bundles were increased almost by an order of magnitude by increasing the particle impact velocity by a factor of two. Numerous crater-like pores on the ends of the carbon rods were formed by alumina particle impaction, and the tips of the fibers in the carbon rods were almost flat and lower than the surrounding matrix. Heating caused by the particle impact increased the thermal oxidization and hence the overall ablation rate of the composite.