The fatigue resistance of gray cast iron is shown to be strongly dependent on graphite morphology and the strength of the steel-like matrix. Considering graphite flakes in gray iron as internal notches, a comparison is made of the fatigue resistance of gray irons and steels of comparable composition, hardness, and microstructure. Application of a Neuber analysis, previously employed in geometrically notched members to relate nominal stresses and strains to local stresses and strains at notch roots, produces quantitative values of the fatigue notch factor, Kf, for various graphite morphologies, matrix structures, and hardnesses. Fatigue resistance of gray irons is enhanced by decreasing graphite flake size. Matrix hardness is of greater importance than structure in determining the fatigue resistance.