The effect of oxygen concentration dissolved in liquid lead-bismuth eutectic (LBE) on the low cycle fatigue properties of T91 ferritic-martensitic steel has been investigated under different temperatures, strain rates and total strain ranges, with an emphasis on the role of oxide films in prevention of liquid metal embrittlement (LME). The results showed that the fatigue life in the presence of LBE was strongly temperature-dependent. Specifically, the fatigue life was observed to decrease with increasing the temperature from 160 C up to 350 C, and then show an increasing trend up to 450 C. Only at the relatively high temperature range of 300e450 C, the oxygen concentration played a role, and the oxygen-saturated condition was found to increase the fatigue life by a factor of 2e3 in comparison to the oxygen-depleted condition. In addition, a significant oxygen effect was also observed under low total strain ranges, and especially the fatigue life was found to be comparable with that in vacuum under a combination of high oxygen, slow strain rates and low total strain ranges. This can be attributed to protective oxide films that were not damaged under this condition and hence had delayed fatigue crack initiation. Once cracks initiated, however, LME occurred at crack tips, causing fast crack propagation. Thus, the nature that T91 is susceptible to LME was not changed by the oxides. The results indicate that under the optimum operation conditions, i.e. adequate LBE oxygen levels, slow strain rates and low applied cyclic deformations, the fatigue life degradation of T91 exposed to LBE can be kept to a minimum.