Various cladding materials consisting of plain carbon steels with lower carbon concentrations and alloy steels with higher carbon concentrations have been considered to analyze the behavior of interdiffusion across the interface theoretically. The combination of the plain carbon steel and the alloy steel for the cladding was selected to make the chemical potential of carbon greater in the plain carbon steel than in the alloy steel. For the selection, the phase diagrams were computed with Thermo-Calc. The theoretical analysis was carried out using DICTRA that is the software developed at the Royal Institute of Technology in Sweden. The analysis indicates that the up-hill diffusion of carbon spontaneously occurs from the plain carbon steel to the alloy steel during annealing at elevated temperatures. For example, the cladding material composed of a plain carbon steel with a concentration of 0.046 mass%C and a 18Cr-8Ni stainless steel with a concentration of 0.06 mass%C exhibits such an up-hill diffusion at 1200 K, resulting in precipitation of Cr23C6 at the cladding interface.