Effects of neutron irradiation and thermal aging on the tensile properties and Charpy impact properties of oxide dispersion strengthened (ODS) ferritic steels for advanced nuclear systems were investigated and discussed with the results of microstructural observation. After the neutron irradiation in JMTR, significant hardening after irradiation at 290 and 400 °C as well as thermal aging at 500 °C, while no effect was observed after irradiation at 600 °C. While the irradiation hardening was not accompanied by a reduction of total elongation in tensile tests, Charpy impact energy at room temperature was reduced after the irradiation. The hardening after the irradiation at 400 °C and the aging at 500 °C was probably due to the formation of Cr-rich phases. The irradiation hardening observed in the ODS ferritic steels irradiated at 290 °C was well explained by the formation of dislocation loops.