The present study focuses on the high temperature oxidation of a AISI 304 chromiaforming nitrided alloy. Isothermal oxidations were performed in air, at 800°C. The effect of nitridation on the steel surface depends on the temperature of the treatment. It leads whether to a γN solid solution formation or to CrN formation. In situ X-ray diffraction has been used to follow the oxides formation. Results show the concomitant growth of CrN and Fe2O3 at the beginning of the test. Then, Cr2O3 quickly appears which leads to the formation of a protective oxide scale (a parabolic rate law is observed). Our conclusions suggest that nitridation increases the high temperature oxidation resistance of 304 steels at 800°C.