We have succeeded in observing Fe-ion-beam induced nanostructural change of MA956 oxide dispersion-strengthened (ODS) steel using anomalous small-angle X-ray scattering (ASAXS) diffractometry. The ASAXS curve shows a power-law scattering that follows the Porod's law and an overlapped shoulder peak at the magnitude of scattering vector, Q, around 0.6 nm . As the X-ray energy approaches the absorption K-edge of Cr (5.990 keV), the intensity of the power-law scattering slightly decreases, but that of the shoulder peak remarkably increases. From these results, the power-law scattering is attributed to Cr-poor scatterers with a radius much larger than 5 nm, and the shoulder peak is attributed to Cr-rich α′ precipitates with a mean radius of 2.8 nm. After the ion-beam irradiation, intensity of the power-law scattering is nearly doubled, but the shoulder peak remains unchanged within an error of 2%. The latter result indicates that the irradiation-induced changes of radius and inter-precipitation distance of the α′ precipitates are smaller than 10%, and the change of their interface thickness is less than 0.5 nm. In this way, our ASAXS results guarantee an absence of irradiation-induced nanostructure change of specific kinds of precipitates together with statistical errors, while observing changes of other precipitates. −1