The article deals with the formation and spread of cracks from thermal fatigue in metallic forms. For a detailed examination of the problem, it is necessary to describe the thermal side of the process and the mechanism of mechanical stressing of the material. For the heating of the material is valid where the thermal diffusivity of the material is exercised by the decision method. The stress of the material by heating from the die casting metal depends mainly on the physical properties of the material when the calculated stress does not exceed the yield point. Also, with the help of dislocations write the life to the origin of the cracks in the cycles eventually the share of this lifetime on the total lifetime in dependence on the mechanical and physical properties of the material. During operation tests the occurrence of the cracks on the mold surface was initiated by the inclusion or concentration of stress in the knurling effect of fine grooves after grinding. Other cracks occurred in the undersurface layer and their direction was statistically accidental.