Mixed mode I and mode II fracture toughness values of five grades of nuclear graphites were measured at room temperature using a centrally notched disk specimen. The specimens were compressed diametrically with a crosshead speed of 0.125 mm/ min in ambient air. The fracture loads of the nuclear graphites decreased as the notch inclination angle increased under mixed mode I-II loading. The nuclear grade graphites with the higher bulk density and coarser coke particles showed higher mode I and mode II fracture toughness. The ratios of KIIc/KIc were in the range of 1.4 ~ 1.5 regardless of the coke type and size

Keywords: Nuclear grade graphite, Notched disk specimen, Mixed mode I-II loading, Fracture toughness, Bulk density, Coke particle.