Nitriding and nitrocarburising treatments are well accepted methods of improving the wear performance of tool and die steels. However, our understanding of the relationship between nitriding process parameters, microstructure and fracture behaviour of the surface layers is far from complete. Vickers hardness indentations generate radial fractures in brittle surface layers, and it has been shown that the length of these cracks can be used to provide valuable information about the fracture toughness of these layers. This paper describes an investigation of the application of indentation fracture testing to nitrided and nitrocarburized HI 1 hot work tool steel. The results suggest that where a sufficiently thick compound layer has formed, this method has the potential to be applied as a pseudo non-destructive method of monitoring the fracture properties of treated surfaces on actual tool parts.