Abstract: Cu-Ni-Cr-Mn cast steel is a kind of low alloyed high tensile steel, which is used in ship industry. It often become worthless due to the crack defect. An investigation on the particularity and the causes of crack formation in this steel has been carried out with optical microscope, scanning electric microscope, microprobe and Auger spectroscope. Results show that the crack is a intergranular brittle fracture caused by eutectic phases γ Fe+ (Fe, Cr, Mn) ₃C precipiting at the austinite grain boundaries because of solidification microsegregation. With broadening solidification interval and decreasing the partition coefficient, caused by arbon, segregation index C_max/C_min of the alloying element, such as Cr. ncrease consequently In presence of sulfur, the segregation index of Cr is increased by sulphide formation. In order to prevent the precipitation ofeUtectic phase and eliminate the crack,the concentration of carbon and sulfurmust be decreased and the increase of cooling rateof castings will be helpful.