Fracture toughness is one important mechanical property of reduced activation ferritic/martensitic (RAFM) steels which are primary candidate structural materials applied to fusion reactors. Temperature effect on fracture toughness of the Chinese low activation ferritic/martensitic (CLF-1) steel was investigated in the range of 25–550 °C with miniature three-point bend (3 PB) specimens, using the digital image correlation (DIC) method to measure load-line displacement. Results show that the fracture toughness J0.2(B) of CLF-1 steel decreases from 25 °C to 450 °C and increases from 450 °C to 550 °C. This changing trend with temperature is similar to that of some commercial ferritic/martensitic (F/M) steels and consistent with the temperature dependence of its ductility which is total elongation obtained from tensile testing. The fracture toughness minimum at 450 °C could be attributed to the deterioration of ductility, where the fracture surfaces with few typical dimples indicated quasi-cleavage-like features.