Abstract: Investigates the change of microstructure of the degenerative layer and the hardened layer in high speed milling the high hardness and strength alloy steel. The average temperature when milling high hardness and high strength alloy steel in high speed is studied. The distribution of the micro-structure and the micro-hardness in work-pieces sub-surface layer are also analyzed in different milling parameters. The results show that obvious changes happened in the micro-structure and the micro-hardness of the sub-surface layer and the microstructure of the degenerative layer and the hardened layer depth are affected greatly by the cutting heat. With the improving of the average temperature of the cutting zero, the depth of the degenerative layer becomes deeper. The main microstructures of the degenerative layer is plastic deformation, tempered sorbite and troostite. When the average temperature is high enough, the hardened layer that has the characteristics of self-quenching and plastic deformation is formed in work-pieces surface layer.