High temperature oxidation tests from 600 to 1400°C were carried out for graphite/B₄C/SiC/ZrB₂ composite in different atmospheres such as dry air, dry oxygen and moist air with different moisture contents. From weight change, SEM observation, change of surface roughness and X-ray diffraction of the sample before and after the oxidation, it was found that the oxidation behaviour depends strongly on the nature of oxide layer formed on the surface by oxidation, and that the nature of the oxide layer depends on the temperature and time of oxidation, the oxygen partial pressure, and moisture content. The oxide layer was found to consist of mainly a B₂O₃-SiO₂ glass phase, rich in B₂O₃ at low temperatures and rich in SiO₂ at high temperatures. Active oxidation was observed at about 1100°C in dry air, at about 900°C in dry oxygen and moist air. Particularly, in the presence of moisture, oxidation proceeded extensively and sample was oxidized deeply from the surface to the inside through the less protective layer. An excellent protective layer formed at 1200°C oxidation and it protected the sample from further oxidation under all conditions examined.