Abstract: A 9CrODS steel was prepared by mechanical alloying and hot isostatic pressing methods. The microstructures were characterized by highresolution transmission electron microscopy and highangle annular dark field scanning transmission electron microscopy (HAADFSTEM). Helium ions of 4×1017/cm2 were respectively implanted into the 9CrODS steel and nonODS Eurofer97 steel with a 400keV ion accelerator， and the sizes of helium bubbles in the two steels were observed by HAADFSTEM. The results show that the average grain size of the 9CrODS steel is about 05μm. Two kinds of precipitates were observed in the ODS steel: highdensity nanosize Y2Ti2O7 and large Mn(Ti)Cr2O4 formed mainly at the grain boundaries. The size of the helium bubbles in ODS steels is much more smaller than that in Eurofer97 steel， which means that the nanosize precipitates are helpful to suppress the formation of coarse helium bubbles.

Key words: ODS steel, helium ion implantation, precipitate, YTiO, Mn(Ti)Cr2O4