Abstract: The development of advanced cladding material with improved service performance is a key issue in engineering applications of sodium-cooled fast reactors. At present, the cladding materials of sodium-cooled fast reactors are mainly AISI type 316 or 15-15Ti austenitic stainless steel obtained by the traditional smelting method. However, the high-temperature mechanical properties and neutron irradiation resistance of these current austenitic steels cannot meet the service performance requirements for cladding of commercial fast reactors. Oxide dispersion strengthened (ODS) austenitic steel is considered to be an important candidate material for cladding application in most Generation IV reactors because of its good high-temperature mechanical properties and excellent irradiation resistance. In this study, 15Ni‒15Cr ODS austenitic steel was prepared by mechanical alloying, hot isostatic pressing, and forging processes. As the reference material, 15Ni‒15Cr austenitic steel without oxide addition was also prepared by the same processes. The microstructure of the sample was characterized by high-resolution transmission electron microscopy combined with a high-angle annular dark field. The average grain size of 15Ni‒15Cr ODS austenitic steels is only 0.5 µm, which is smaller than that of the reference material 15Ni‒15Cr (i.e., 0.75 µm). The oxide-dispersed particles distributed in 15Ni‒15Cr ODS austenitic steel are mainly δ-Y₄Zr₃O₁₂ and a small amount of Al₂O₃. The average particle size of oxide-dispersed particles in 15Ni‒15Cr ODS austenitic steel is 12.8 nm, the number density is 5.5×10²² m⁻³, and the interparticle spacing is 26 nm. Compared with the reference material 15Ni‒15Cr, 15Ni‒15Cr ODS austenitic steel exhibits higher strength, particularly at high temperature, which can be attributed to the refinement of crystal grains and the pinning effect of oxide-dispersed particles on dislocations. However, the plasticity of 15Ni‒15Cr ODS austenitic steel decreases at a high temperature of 700 °C. The fracture surface of 15Ni‒15Cr ODS austenitic steel at room temperature shows typically ductile fractures, whereas that at the high temperature of 700 °C shows ductile–brittle fractures.