The ferritic-martensitic steel F82H is a primary candidate low-activation material for fusion applications, and it is being investigated in the joint U.S. Department of Energy-Japan Atomic Energy Research Institute (U.S. DOE-JAERI) collaboration program. As part of this program, two capsules containing a variety of specimen designs were irradiated at two different temperatures in the Oak Ridge National Laboratory (ORNL) High Flux Isotope Reactor (HFIR). The bottom and top parts of these capsules were loaded with disk-shaped compact tension [DC(T)] specimens that were used for fracture toughness characterization. This small (12.5-mm-diam and of 4.6-mm-thick) DC(T) specimen was developed at ORNL for testing irradiated materials. Six specimens were irradiated in each “low-”and “high-” irradiation temperature capsule up to ∼3.8 dpa. Irradiation temperatures were measured by thermocouples. In the low-temperature capsule, three specimens were irradiated at an average temperature of 261°C and another three at 240°C; temperature variation during irradiation was within ±19°C for a given specimen. In the high-temperature capsule, all six specimens were irradiated at an average temperature of 377°C in the bottom part of the capsule; temperature variation during irradiation was within ±30°C for a given specimen. All irradiated specimens failed by cleavage instability. From these data, fracture toughness transition temperatures were evaluated for irradiated F82H steel and compared to unirradiated values. Specimens irradiated at the higher temperature exhibited a relatively modest shift of the fracture toughness transition temperature of ∼57°C. However, the shift of fracture toughness transition temperature of specimens irradiated at 250°C was much larger, ∼191°C. These results are compared with available tensile and impact Charpy data for this material.