Abstract: In the possible accidents, the nuclear graphite used in high-temperature gas-cooled reactor may be oxidized. 2-D scanning electron microscope observation and 3-D CT scanning analysis on the oxidized graphite were conducted to study the surface and inner micro-structure of the graphite. After the oxidation, there appear big voids inside the graphite, which makes the probability distribution of grayscale change from single-peak to double-peak. By analyzing the mean grayscale and the mean density of layered graphite versus the depth of the graphite, it is found that the oxidized zone of the graphite is mainly within 1 mm under the surface. When the total mass loss is small, the superficial zone suffers more oxidizing mass loss, and the crack network caused by oxidation grows into the graphite when oxidizing process continues.