Abstract: Microcrack defects in the center of H13 Hot working die steel were detected by ultrasonic inspection and macrostructure test. In this paper, the cause of microcrack die steel was analysed by optical microscopy,scanning electronic microscopy and energy dispersive spectrometer.The results indicated that the severe microstructure segregation formed in ingot solidification process and internal stress caused by the large amount of brittle Al₂O₃,Al₂O₃-MnS compound inclusionsand blocky eutectic carbidesin thesegregation bandswere the causes of micro-cracks in the center area.