The low cycle fatigue properties of T91 have been tested in LBE with oxygen concentrations ranging from 1.16x10-6 to 6x10-10 at 350°C. The results showed that the fatigue life of T91 in a low oxygen concentrated LBE was drastically reduced compared to the lives obtained in a vacuum at the same temperature. The crack propagation modes revealed that the crack propagated mainly transgranularly through the prior austenite grains and laths. No obvious plastic shear strains was present at the crack tip when LME occured. However highly localized strains were detected in the vicinity of the crack tips for specimen tested in vacuum.