The oxidation behavior of NF616, a ferritic-martensitic (F/M) steel, in supercritical water (SCW) has been investigated because of its potential applications in future Generation IV nuclear reactor systems. Shot peening is being used to significantly reduce the grain size in the near-surface regions of this steel and hence altering the diffusion characteristics of elements in the modified surface layer during high temperature exposure. This refined grain structure is envisioned to improve the corrosion resistance of NF616 without changing the bulk properties of the metal.  After exposure to SCW with a test section inlet dissolved oxygen content of 25 ppb at 500°C for up to 336 hours, both as-received (AR) and shot-peened (SP) samples formed a duplex oxide layer, including an iron-rich outer magnetite oxide layer and a chromium-rich inner oxide layer consisted of spinel and ferrite.  A larger iron/oxygen ratio was observed in the outer oxide layer of the shot peened samples, indicating enhanced diffusion of iron during the growth of the oxide layer.  Chromium diffusion was also notably enhanced for shot peened samples due to the finer grain size in the near-surface region.