Abstract: The relationship between grain size and cleavage fracture toughness K_IC, i.e., conditional fracture toughness K_Q, was investigated for a high speed wheel steel with 0.5% C by compact tension testing. It is indicated that the grain size has an evident effect on the fracture toughness. The fracture toughness is more dependent on the largest grain size than the average grain size of the C50 wheel steel. When the average size of the 5% largest grains is in the range of 30 to 73 μm, the cleavage fracture toughness K_Q decreases linearly with the logarithm of average size of these largest grains increasing.