Si diffusion layers on SUS430 and SUS304 stainless steel surfaces were formed by the pack-cementation method. Thickness of the Si diffusion layer became nearly constant value after 20 ks of the treatment time. The thickness of the diffusion layer for SUS430 steel was about 200 μm, while that for SUS304 steel was about 100 μm. For both the steels, the Si diffusion layer consisted of α-phase, and the Si concentration in the surface region was about 6 mass%. High temperature corrosion tests of these samples were carried out under a coating of molten NaCl-KCl-Na<sub>2</sub>SO<sub>4</sub> in an 18.4 vol%O<sub>2</sub>-20 vol%H<sub>2</sub>O-1348 vol. ppm HCl-N<sub>2</sub> atmosphere at 1173 K. As a result, it was found that for both the steels the oxidation mass gains for the pack-cementation treated samples were far lower than those for the no treated samples. Corresponding to the results, the thickness of a scale formed on the treated samples was thinner than that on the no treated samples. In the treated samples, further, the formation of SiO<sub>2</sub> layer was observed in the interior side of the scale.<br>