Disclosed is a method of predicting damage of dies for plastic processing of metallic materials, typically, forging dies, by predicting brittle fracture ("great crack" or "initial crack") dominating die lives contribute to die design including choice of materials, hardness and configuration of the die. The method is characterized in that the die design is carried out by choosing the condition that none of the anticipated values of brittle fracture, F c1 to F c3 , calculated by the formulae 1 to 3 below exceed the critical values depending on the material used. [ formula   1 ]   F c 1 = ( σ m / σ e q ) [ formula   2 ]   F c 2 = ( σ m / σ 1 max ) [ formula   3 ]   F c 3 = ( σ 1 max / σ e q )
σ m : mean normal stress loaded to the tensile side of the
die
σ eq : Von Misese's equivalent stress
σ 1max : maximum principal stress