The in-plane bending fatigue tests were carried out on FCD 450 at elevated temperatures up to 723 K. The threshold stress intensity range ΔKth and its effective values ΔKeffth were observed as a function of the temperature T. Plots of ΔKeffth and T revealed that ΔKeffth remained constant up to 623 K and then decreased to a value about 28% lower at 723 K. The ΔKth-T relations showed that ΔKth remained constant in the range of 473 K to 723 K, its value being about l0% lower than that at room temperature. While the ΔKeffth value at 723 K decreased by 28% lower than that at 623 K, there were no significant differences between the ΔKth value at 723 K and that at 623 K. It appears that the rise of closure level Kcl induced by the high-temperature oxide on fracture surface results in the apparent increase in ΔKth value at high temperature such as 723 K.