In this study, the stability of grain size and oxide nanoparticles in the ODS steel upon annealing at high temperature (650-1350 A degrees C) has been evaluated. The ODS Fe-Cr-W-Ti-Y2O3 steel has been manufactured by powder metallurgy, consolidated by hot isostatic pressing and processed by hydrostatic extrusion. Such a processing brings about ultrafine grain structure reinforced with oxide nanoparticles (few nm in diameter) and results in superior mechanical properties. The stability of nano-oxides has been analyzed by small angle X-ray scattering together with transmission electron microscopy. The results obtained revealed excellent thermal stability of ultrafine grained ODS ferritic steel, which was attributed to the resistance of oxides against coarsening.