A nanostructured surface layer with a mean ferrite grain size of ～8 nm was produced on a Fe-9Cr steel by means of surface mechanical attrition treatment. Upon annealing, ferrite grains coarsen with increasing temperature and their sizes increase to ～40 nm at 973 K. Further increasing annealing temperature leads to an obvious reduction of ferrite grain sizes, to ～14 nm at 1173 K. The annealing-induced grain refinement is analyzed in terms of phase transformations in the nanostructured steel. A nanostructured surface layer with a mean ferrite grain size of ~ 8 nm was produced on a Fe-9 Cr steel by means of surface mechanical attrition treatment. Upon annealing, ferrite grains coarsen with increasing temperature and their sizes increase to ~ 40 nm at 973 K . Further increasing annealing temperature leads to an obvious reduction of ferrite grain sizes, to ~14 nm at 1173 K. The annealing-induced grain refinement is analyzed in terms of phase transformations in the nanostructured steel.