Abstract Spacecraft structures in low earth orbit encounter a degradation problem caused by the impact of atomic oxygen (AO) in the space environment. This paper presents an experiment of impact degradation on the ground. The experiment was carried out in a vacuum space chamber. AO produced at a plasma torch was accelerated fluid-dynamically with argon (Ar) working gas. As the target materials, several carbon-based materials were employed; i.e., graphite, carbon fiber/carbon composite (C/C composite) and silicon impregnated C/C composite (Si-C.C.). The degradation was analyzed in the aspect of macroscopic mass loss per unit area and the microscopic degradation modes. The temperature effect and the exposure time effect are investigated and the comparison among the target materials is developed. The mass loss per unit area is approximately proportional to the exposure time and depends on the temperature of material surface. The eroded surfaces of materials were observed with a scanning electron microscope and an electron probe micro-analyzer. In case of C/C composite, the matrix region erodes more than carbon fiber, however, this effect is more pronounced in the normal oxidation at the ambient atmosphere. The mass loss of Si-C.C. is less than that of the corresponding C/C composite and only the C/C composite region erodes deeply with almost no erosion in Si and SiC regions.