Reduced Activation Ferritic/Martensitic alloys are being developed for application in future thermonuclear fusion reactors. The behavior of the reduced activation martensitic steel F82H-mod was compared with small amounts of related alloys JLF-1, JLF-1B and ORNL-9Cr2WVTa, following irradiation in the High Flux Reactor (HFR) in Petten. Tensile, KLST-type Charpy impact and CT fracture toughness mechanical properties specimens, were neutron irradiated to a dose level of 2–3 dpa at 300°C. Results of post irradiation tensile, miniaturized charpy impact, and static fracture toughness tests are presented and interpreted in terms of irradiation hardening and irradiation induced shift of ductile to brittle transition temperature (DBTT) and reduction of upper shelf energy (USE). The observed irradiation hardening (140 – 170 MPa at 300°C) is moderate, whereas the observed radiation induced shift in DBTT is approximately 150°C for KLST-type impact test specimens for F82H. Similar observations are made for the static fracture toughness tests. For JLF-1B the irradiation hardening is one third higher than for JLF-1, and shift in DBTT for JLF-1B is almost 70% larger than for JLF-1. The difference is attributed to the tenfold larger boron content prior to irradiation.