Abstract: The SEM,TEM and EDS techniques and the deep corroding method were applied to analyze the microstructure and elemental profile of the inter-diffused layer between the Mo-3Nb single crystal and CVD tungsten coating. It is showed there is an inter-diffused layer between the Mo-3Nb single crystal and CVD tungsten coating, the crystalline structure of the inter-diffused layer is roughly as same as the Mo-3Nb bulk and W coating, the thickness of the inter-diffused layer is about 10μm and the heavy inter-diffused layer is about 3 μm. Additionally, it is found that carbon and oxygen are enriched in inter-diffused layer. The deep corroding method is an effective way to analyze the microstructure of the Mo and Nb single crystal.