[en] In present study, we evaluated the irradiation properties of high Cr ferritic/martensitic steels. These steels were irradiated in HANARO for 14 days at 297±5 .deg. C, and for 15 days at 307±5 .deg. C, and then the fluence was 2.9x10²⁰n/cm² (E>1.0Mev). High temperature tensile test after irradiation was performed at hot cell in IMEF. Tensile test temperature range was from room temperature to 700 .deg. C. The yield and ultimate tensile strength of specimens increased, and the elongation of specimens decreased by neutron irradiation. Especially elongation was greatly decreased. As the tensile test temperature increased, the increase of strength by irradiation was diminished. But elongation was not recovered at high temperature tensile test. As the irradiation fluence increased, the increase of yield and tensile strength became larger. But the elongation was not influenced by the increase of irradiation fluence