Herein, a study on radiation-induced defect structures in a reactor Fe–14Cr–1W (wt%) steel with grain sizes of 5 μm and 110 nm is presented. Self-ion irradiation of the steel in both states is conducted with a damage dose of 10 dpa. Microstructure characterization shows that the density of intragranular radiation-induced dislocations is significantly lower in case of the nanostructured material. From the results obtained, it follows that microstructure refinement to a grain size of about 100 nm can be effectively used to approach a problem of higher defect production rate in Fe–Cr steels driven by irradiation and to produce alloys with significantly enhanced mechanical performance as well as radiation tolerance.