The oxidation behavior of Cr- and Si- bearing ferritic stainless steels has been studied in the O
2+H
2O+Ar mixed atmospheres at 600°C, in an attempt to differentiate the effect of O
2 and H
2O on the occurrence and growth of nodular scale and to clarify the effect of Cr and Si contents on the oxidation resistance. In the results, the reddish brown scale grew on the SUS 430 steel in above 10% H
2O containing atmosphere regardless of O
2 concentration. On 11%Cr-2%Si steel, the reddish brown scale developed in the above 15% H
2O and under 10% O
2 mixed Ar atmospheres. On the other hand, 18%Cr-2.5%Si steel showed good resistance to reddish brown scale in all conditions examined. It has been observed that O
2 promotes the formation of Cr-enriched M
2O
3 type scale suppressing the nodular scale formation. However, once nodular scale is formed, O
2 accelerates its growth. While, it has been found that H
2O promotes the formation of M
3O
4 type scale and causes nodular scale.
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