The structural evolution of the oxide films at 25-450 degrees C on nanocrystalline (NC) and coarse crystalline (CC) 304 stainless steels (SS) was investigated. The structure of the oxide film on both NC and CC SSs was observed to undergo transient processes from a bi-layer to a single-layer and then back to a bi-layer when the temperature changed from the low range (25-150 degrees C) to the medium range (150-300 degrees C) and subsequently to the high range (300-450 degrees C), respectively. These formation mechanisms of the oxide films on SS were attributed to the different diffusion properties of Cr and Fe in the three temperature ranges. The thickness of the oxide films was similar between the NC and CC SSs below 300 degrees C due to their similar Cr-ox/Fe-ox concentration ratios in their oxide films at this temperature. Above 300 degrees C, Cr diffusion enhancement in the NC matrix led to a higher Cr-ox/Fe-ox ratio and better compactness of the oxide film, which resulted in a slower atomic diffusion rate in the oxide film and a thinner oxide film. Therefore, the temperature of 300 degrees C was concluded to be the critical temperature of the detectable Cr diffusion enhancement in the NC SS compared to the CC SS. (C) 2017 Elsevier B.V. All rights reserved.