The concept of “Reifegrad” is widely used for estimating the quality of gray cast iron. According to this concept, quality is expressed by the comparison of the actual tensile strength with the standard tensile strength of the same carbon equivalent. The tensile properties of ductile cast iron are basically governed by matrix structure and also affected by nodularity of graphite, existence of microporosity, etc. Because the Brinell hardness properly expresses the state of matrix structure, the comparison of the actual tensile properties and standard properties for the same Brinell hardness is considered to be “Reifegrad” of ductile cast iron. Estimation of the quality of ductile cast iron was attempted in this way in several case were made trial.