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Effect of microstructural changes on the crack generation and propagation during low cycle fatigue was investigated on hot working tool steel SKD6, having the hardness of 48HRC, at 873K in air. The stress amplitude vs. number of cycles relationship under constant strain condition consisted of three stages, as follows. The first stage is a short period of work hardening, and the second stage is dominated by cyclic softening due to dynamic recovery of matrix structure and coarsening of carbides. At the final stage, stress amplitude is drastically decreased by crack propagation, which could result from micro-cracks initiated at scales near surface.