The present paper attempts to describe a study concerning the influence of mean stress or R-ratio and microstructural variation, percentage polygonal ferrite, on the near threshold fatigue crack extension characteristics of a flake graphite cast iron. Basically significant effects of R-ratio and microstructure on the fatigue threshold stress intensity range, ΔK t h , were observed. Also at intermediate ΔK levels the flake cast iron microstructure exhibited a much higher slope in the two-parameter fatigue crack growth relationship than those observed in ductile low alloy steels.It was also observed that at low R-ratio the ΔK t h values of the flake cast irons were significantly larger than those recorded for iron-based and low alloy steel materials. The high levels of ΔK t h were explained in terms of roughness induced crack closure and crack deflection processes which resulted from the very rough topography of the fatigue fracture surfaces. Finally the role played by the graphite constituent in the fatigue fracture processes prevalent in cast iron microstructures was discussed.