Decarburization treatment was carried out on SKD5 tool steel by heating at 1323K for maximum 5.4ks and quenching followed by tempering at 873K for 3.6ks. This resulted in decarburized layer of maximum thickness 0.45mm. After the treatment, WC-Co cermet of 0.2mm thickness was deposited by high-velocity oxygen fuel thermal spraying. Tensile tests were carried out on the specimens, where the load was applied parallel to the specimen. The results show that the presence of decarburized layer increases the interfacial fracture toughness. The reason seems to be that the large plastic deformation of decarburized layer relieves shear and tensile stresses at crack tip. Edge indent tests were also conducted, where shear delamination of coating occurred during the penetration of indenter. The result is that the delamination energy decreases due to the presence of decarburized layer, which is contrary to that of the tensile test. The shear fracture of soft decarburized layer may be the reason for the smaller delamination energy of decarburized specimen.