Abstract: The saw cutting surface appeared unevenness problem of the 718 plastic mould steel. The reason for the uneven surface was analyzed by chemical composition test, microstructure morphology observation, micro-area composition analysis and hardness test. The results show that the segregation of chromium, manganese and molybdenum was primary cause of the uneven saw cutting surface. The segregation of element made the 718 plastic mold steel form a hard mirostructure composed of tempered martensite and tempered lower bainite, which was harder than that of the hard microstructure of the substrate, leading to the unevenness of the surface during the cutting process. It was suggested that reducing the segregation by increasing the cooling strength, or adding the homogenization heat treatment process to avoid this problem.