It is said that the key parameters of the strength of graphites are the pore and grain sizes, and many researchers have been investigating on the brittle fracture of graphite. To investigate a relationship between the fracture toughness and the microstructure three kinds of graphites with different grain size distributions but having almost the same porosity were prepared, and the fracture toughness at room temperature was measured. Fracture toughness, K_<in> and K_<max>, of specimens contained with coarse grains was increased in comparison with a fine grained graphite. The K_<in> and K_<max> showed different relations with coase grain fractions. The distribution of K_<in> with coarse grains, derived from the load which seems crack growth started, was not affected by the fractions. On the other hand, the distribution of K_<max>, derived from the maximum load, were affected by the fractions.
