To improve the fatigue reliability for spheroidal graphite iron, FCD370, by WC plasma-sprayed coating, the influence of the surface modification on fatigue fracture mechanism was experimentally studied. The fatigue properties of FCD 370 are found to be obviously improved by WC plasma-sprayed coating, so that they can no longer be estimated by using a concept of "S-N Globe" which has successfully been applied to many surface-treated materials. Through fractography using SEM, it was found that the fatigue crack initiated at the debonding graphite at the interface between the substrate and sprayed coating. Furthermore the meso-crack growth was considerably influenced by the graphite due to its irregular profile. A "P-S-N Globe" was aplied by combining the experimental analysis with the linear notch mechanics and reliability engineering.