Abstract：The cracks perpendicular to the concaved surface on continuous casting bloom caused the inclined cracks on the surface of rolled bar of high carbon chromium bearing steel GCr15 (eqv. AISI 52100). The origination of the cracks on the bloom has been studied by the methods of macrostructure etch test, microstructure test and fractography SEM-EDS analysis of pre-fabricated notch specimens. The results show that the cracks extend along the columnar crystal boundaries, and there are carbide networks and severe micro-cavities on the grain boundary. Exposed dendrites with smooth surface appear in the shrinkage cavities in more than 2 mm diameters. The cracks originate from the formation and coalescence of the severe shrinkage cavities during solidification due to a lack of molten metal, which are caused by seriously weak cooling.