In the present work, a performance of ASTM A439 Austenitic Ductile Iron type D5S at high temperature in the oxidizing environment was investigated. The obtained results revealed that exposure at temperatures 800?C, 850?C, and 900?C resulted in relatively high mass gain and an extensive oxide scale spallation from the samples? surfaces during cooling. However, the material exposed at 950?C revealed a better oxidation resistance and no oxide scale spallation. The material exposed at 1000?C showed the best oxidation resistance among the studied samples. The surfaces and cross-sectional investigation revealed that the material exposed at 950?C formed mostly Ni/Cr/Mn-mixed protective oxide scale and local formation of Fe-rich nodules. In comparison with the sample exposed at 1000?C, a smaller amount of Fe-rich nodules per area unit was observed and most of the surface was covered by Ni/Cr/Mn-mixed protective scale. The latter was explained by the change in the calculated diffusion coefficients in the alloy for Ni and Fe, namely up to 900?C the diffusion coefficient for Fe was much higher than for Ni, while above 900?C the diffusion coefficient for Ni becomes higher than for Fe. This phenomenon was correlated with a phase transformation from ?-Fe into ?-Fe resulting in the diffusion coefficient change.