The effect of mean stress on the fatigue strength of gray cast iron was examined and its mechanism was investigated. Fatigue endurance limits were experimentally obtained by round bar fatigue tests under various stress ratios. The critical stress amplitude for the fatigue endurance limit decreased with an increase of mean stress, but its dependence on mean stress was not the linear relation that the modified Goodman relation suggests. A fractography technique was applied to the fractured specimen in different stress ratios. The fracture appearance in the stress ratio range −1 ≤ R < 0.6 suggested that the fatigue crack initiated from large size graphite and propagated to the Pearlite matrix, whereas that in the stress ratio range R ≥ 0.6 showed a static fracture feature of dimple pattern. The fatigue endurance limit in the stress ratio range −1≤ R < 0.6 was analyzed by fracture mechanics. The relation between mean stress and stress amplitude was calculated from the threshold value of effective stress intensity factor, ΔK_eff,th with an assumption of an initial crack size similar to the maximum graphite length of the material. The estimated relationship, which represents the mean stress effect on the stress amplitude of fatigue endurance limit, quantitatively agreed well with the experimental results, in spite of there being no references to the fatigue endurance properties of the material.