Oxide dispersion strengthened (ODS) steel has excellent high-temperature performance and corrosion resistance. It has broad application prospect and development space in the key field of high temperature structural materials for nuclear power. 9Cr-ODS steel has become one of the most promising candidate materials in advanced nuclear reactors because of its excellent high temperature mechanical properties and radiation resistance. In this work, 9Cr-ODS steel was designed and prepared by powder metallurgy process. The as-hot isostatically pressed (HIPed) microstructure of the steel was studied and analyzed, including matrix grain distribution characteristics, micron-scale large size precipitated phase, and nanoscale oxide particles. In addition, the high temperature microstructure thermal stability of 9Cr-ODS steel aged at 650℃ for different time was researched by means of XRD, SEM, TEM and hardness test, and the microstructure change of matrix and hardness properties were analyzed. Based on the contrast analysis of the matrix microstructure and hardness properties, the hardness change of the austenitic ODS steel at high temperature was obtained. The results showed that the original as-HIPed microstructure of 9Cr-ODS steel is mainly composed of martensite lath and large amount of Y2O3. During ageing process, the lath martensite of 9Cr-ODS steel gradually coarsens and the number of dislocations decreases with ageing time increasing, and the Cr23C6 carbides begin to precipitate along the grain boundary and grow up. At the same time, the Laves phases with large size begin to precipitate in ageing and then grow with the increase of ageing time. Meanwhile, ageing treatment makes Y2O3 phase with larger size further grow, while Y2O3 phase with smaller size precipitate increase. This phenomenon can probably be associated with the dissolution of the fine particles induced from the particle coarsening, generally called the Ostwald-Ripening mechanism. The change of microhardness during ageing was related to the size of lath martensite and the number and density of the second phase precipitation, especially Cr23C6. The hardness test results show that the microhardness first decreases and then tends to be stable with the increase of ageing time.