With regard to the stress-strain curves of gray cast iron the following results have been obtained. In the range (i), that is from room temperature to about 500°C, the tensile deformation curves of ferritic and ferrite-pearlitic gray irons show the linear relationship between stress and strain at lower deformation stress and high strain hardening rate at higher stress. In the range (ii), from about 500°C to α→γ transformation starting point, the deformation curves show that the strain hardening rate decreases as the temperature increases, ductility increases up to 770°C and brittleness appears above 770°C. In the range (iii), eutectoid transformation range, the flow stress increases with the rise in temperature in ferritic gray iron. In the range (iv), from α→γ transformation finishing point to about 1,100°C, stress-strain curves show deformation with constant flow stress and elongation increases as the temperature is raised. In the range (v), from about 1,100°C to liquidus, the curves in solid state show low stress and slightly higher strain.<br>  With regard to the stress-strain curves of ductile iron the followings have been clarified. In the range (i), from room temperature to about 400°C, the curves show high strain hardening and a kind of nonhomogeneous plastic deformation or the so called “serrated flow” takes place within the range of 150∼400°C. In the range (ii), from about 400°C to α→γ transformation starting point, strain hardening decreases as the temperature is raised. In the range (iii), eutectoid transformation range, the wavy flow, the other kind of nonhomogeneous plastic deformation, takes place in the range of 810 to 845°C and it disappears and strain hardening increases as the temperature rises in ferritic ductile iron. In the range (iv), from the α→γ transformation finishing point to about 1,100°C, the curves show high strain hardening just above the eutectoid transformation range and strain hardening decreases as the temperature is raised till the plastic deformation takes place at constant stress. The wavy flow takes place also at higher temperatures in this range. In the range (v), from about 1,100°C to liquidus, the curves in solid state show the occurrence of inter-cellular fracture as the stress increases and the curves in solid and liquid coexisting state show that the flow stress increases slowly and then decreases till tensile fracture occurs.