A C/Si/O composite material having improved oxidation resistance is prepared by impregnating graphite with a crosslinkable silane or siloxane, causing the silane or siloxane to crosslink within the graphite, and heating at 300-1,200° C. in a non-oxidizing gas. The C/Si/O composite material can be efficiently prepared through simple steps, on an industrial scale and at a low cost and will find use as a high-temperature structural material by virtue of oxidation resistance.