Under severe cutting conditions of carbide face-milling operation, cracks first come out almost perpendiculary to the cutting edge on both rake and relief surface of blades, and next, cracks parallel to the cutting edge appear according to circumstances. These cracks have negative effects on the durability of a cutter and especially when cutting fluids are applied, they occur remarkably and the cutter is damaged at an early stage. Situation and circumstances under which cracks occur have three characteristic features. (1) Cracks occur easily when the rate of cooling is better or the cutting condition is severe, namely the effect of thermal stress which depends on temperature difference of cutter surface is large. (2) Occurrence of cracks has size effect. (3) Cracks occur as a saturation curve. It is reported that probability Pv of cracks occurring by one thermal tensile stress σ_T is given by the following equation and the features of crack occurrence are explained qualitatively. Pv=1-exp [-r_mLdsecc/(K(m+1))(σ_T/(σ₀))^m 2r_m : thickness of carbide tip Ldsecc : size of chip contact area σ₀ : strength const. of tip K : const. depending on relative heat transfer coeff. m : material const.