The concept of the diffusion barrier coating system (DBC system) is summarized and the latest results are presented. The DBC system is comprised of alloy substrate/diffusion barrier/Al-reservoir/an external scale. Diffusion flux (JAl) of Al through the barrier layer will be given approximately by JAl = DAl x SAl x (d CAl/d x), where DAl and SAl are the diffusion coefficient and solubility limit of Al in the barrier layer, respectively as well as d CAl / d x is driving force given by the concentration difference across the barrier (d CAl) divided by the thickness of the barrier layer (d x). A slow diffusion flux can be obtained by using low values of DAl, SAl, or (d CAl /d x). Accordingly, a selection of a barrier layer with lower DAl and SAl is essential. A low driving force is also an important factor, and can be achieved by using lower CAl with a constant barrier layer thickness dx. At higher temperatures, however, the barrier layer can react with the alloy substrate and Al-reservoir layer, resulting in gradual degradation of the barrier layer. This means that the thickness dx of the barrier layer tends to decrease and may finally disappear. With decreasing thickness of the diffusion barrier layer, the driving force (dCAl/dx) will increase, and the effectiveness of the barrier layer will be eliminated. Therefore, it is essential to maintain a constant thickness of the barrier layer for long exposure time. Several types of the DBC system are proposed, a single barrier layer and triple-layers with g + g’ and g’ inserted among these barrier layers.