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One of the most common steels for plastic moulds is the ISO 1.2738 usually delivered after quenchand tempering. The moulds are forged starting from an ingot and usually are heavy sized, beingtheir dimensions up to 1m x 1m x 2m. It is well known that this can cause problems in terms oftoughness, fatigue resistance and structural heterogeneity from surface to core. In order to improvethese features, new pre-hardened non-standard materials have been developed recently. The aim ofthe study is to characterize the fatigue behaviour of these new generation steels using the fracturemechanics approach: crack propagation thresholds as well as stable and unstable crack growthshave been investigated by measuring da dN at increasing or decreasing DK’s; the fracturesurfaces have been observed by SEM microscope. Samples have been machined both from thesurface and core materials of the original bloom. Moreover, a microstructural survey has beenperformed at different distances from the surface and the correlation with the local fatiguebehaviour has been sought after.