Due to excellent high temperature resistivity on the strength, carbon fiber reinforced carbon matrix composites are thought to be one of attractive materials in a nuclear engineering field, e.g. the application to control rod sheath and so on in high temperature gas-cooled reactors as well as plasma facing components in fusion reactors. Prevention of the buckling fracture is placed as one of key subjects in the component design of C/C composites. The buckling fracture test was, therefore, carried out using 2-dimensional C/C composite (2D-C/C composite), and fracture surfaces were observed by SEM. From the buckling fracture test, it was found that there are three kinds of fracture modes; i.e. delamination fracture mode of fiber bundles at low aspect-ratio region, shear fracture mode of fiber bundles at middle aspect-ratio region and shear fracture mode of fiber bundles accompanying by fiber buckling at high aspect-ratio region.