Irradiation hardening and microstructural change of oxide dispersion strengthened (ODS) ferritic steels were investigated by using the ion-irradiation technique. The material used for the present study was K3 (16C-2W-0.3Ti-4.6Al-0.4Y2O3) ODS ferritic steel for the application to Generation IV concept advanced reactors and fusion reactors. 6.4 MeV Fe3+ ions were used to irradiate to the K3 ODS steel by using a 1.7 MeV tandem accelerator. The irradiation temperatures were 300°C and 500°C. The nominal displacement damage rate and total displacement damage were 1×10−3 dpa/s and up to 10 dpa at about 600-nm depth from the irradiated surface, respectively. Nano-indentation hardness was evaluated with a Berkovich indenter. After the ion-irradiation at 300°C up to 1 dpa, the normalized nano-indentation hardness (hardness after irradiation/hardness before irradiation) of the K3 ODS steel reached about 1.28 and the value showed no change up to 10 dpa. On the other hand, the ion-irradiation at 500°C up to 10 dpa showed no significant irradiation hardening. TEM observation revealed that dense and fine dislocation loops were formed in the ion-irradiated steels at 300°C, which were probably enough to explain the irradiation hardening.