This study is focused on the brittle fracture properties after crack extension under large scale cyclic loading for structural steel. Cyclic fracture tests are conducted with 4-point bend specimens with fatigue pre-crack or machined notch. These specimens are differ in stress/strain concentration. It is shown that the brittle fracture resistance under cyclic loading can be lower than that in monotonic loading at a lower temperature. The FE-analysis implies that the accumulation of large plastic strain, that acts as a prestrain, near the crack tip by cyclic loading is responsible for the lower fracture resistance. The deterioration of critical CTOD values under cyclic loading, which appear at more extensive temperature in machined notched specimens, can be quantitatively estimated with the translation of ductile/brittle transition temperature for monotonic loaded specimens with fatigue pre-crack to higher temperature. It seems to be found that this translation approximately correspond to the saturated temperature translation as increasing of uniform prestraining in steel used.