Surface hardening of austenitic stainless steels without degradation of corrosion resistance has been a subject of continuing interest in the field of surface engineering technology. This paper presents a method, using lowtemperature plasma-carburized processing, to produce a hard and corrosion-resistant layer for austenitic stainless steels. The process is based on the diffusion of carbon and/or nitrogen from the surface into the austenitic substrate without forming any carbides or nitrides. The resultant carburized and/or nitrided surface layer shows highly superior saturation of carbon or nitrogen. It is called expanded austenite or S-phase. Important details of the S-phase have not been revealed yet, e.g., the formation of super saturated solid solution at constant processing temperature. For this study, austenitic stainless steel with 3 mass% copper was selected for use as a substrate of low-temperature carburizing. Actually, Cu shows no tendency for formation of carbides in steel. Samples were plasma-carburized using DC plasma apparatus under 667 Pa of mixed gas flow of 5% CH 4 +45% H 2 + 50% Ar at 673 K or 723 K for various durations. The treated S-phase of steel with copper was compared to those of 304 steel and 316, 317 steels containing molybdenum. Results show that copper addition, just as molybdenum addition, similarly enhances carbon super saturation and the surface-layer hardness. Furthermore, the carburized layer depth was enhanced with copper addition as well as Mo addition. The effects of copper and molybdenum are discussed with measurements of the lattice constants in consideration of the size effect.