Abstract: By thermal cycling treatment from 1200℃ to room temperature, a considerable acceleration of Cr diffusion to grain boundaries in Fe25Cr5Al alloy occurred due to nonequilibrium Cr element segregation, and resulted in the precipitation of Cr-rich α'-phase at grain boundaries and within grains; micrometer scale of α'-phase was observed at the grain boundary, the phase transformation resulted in the failure of the alloy by a mode of transgranular deavage fracture.