The effects of composition and process variables on the tensile strength of flake graphite cast iron are well established. However, when compared with most metallic materials, there is little quantitative understanding of the way in which the microstructure controls the mechanical properties. In this paper a fracture mechanics approach is used to develop a mechanistic interpretation of the relationship between the microstructure and tensile strength. Flake graphite cast irons have been studied in which matrix microstructures of pearlite, ferrite and tempered martensite have been developed by heat treatment. For a given eutectic cell size, a linear relationship exists between the tensile strength and the fracture toughness KIC, for all of the matrix structures studied. The tensile strength is interpreted as a brittle fracture stress which is determined by the fracture toughness of the iron and an inherent defect size which is defined by the eutectic cell size.