Thermal barrier coating (TBC) is an essential requirement of a modern gas turbine engine. The TBC failure is the delamination and spallation. The oxidation damage under high temperature results in the reduction of interfacial adhesion. The interfacial fracture toughness is an important property to analyze the TBC failure. The interfacial fracture toughness of ceramic coating - bond coating has been researched in the past. However, the facture toughness of the bond coating - substrate due to the Al depletion was very few studied. In this study, a NiCrAlY bond coating by air plasma spray (APS) was deposited. The substrate was directionally solidified superalloy (DZ40M). Isothermal oxidation was performed at 10500 for 100h. Using the HXZ-1000 micro-hardness equipment, the five different times was chosen to test the hardness and the crack length, and then the fracture toughness was obtained. While the oxidation exposure time increased at 10500, the hardness of the substrate close to the bond coating decreased with the increase of the bond coating’ hardness. Meanwhile, the interfacial fracture toughness of the bond coating - substrate decreased because of the Al depletion.