Abstract: H13 mould steel for aluminium extruded sections cracked after being used for less than 20 minutes.Macro-examination,chemical composition analysis,hardness testing,microstructure examination and fracture analysis were carried out on the cracked mould.The results show that aluminiun swarf come into the grinding cracks produced during improper grinding process and,then temperature of the aluminiun swarf rise because of the rubbing and squeezing effects between aluminiun billet and cavity face,which led to the fusion and volume expansion of aluminiun swarf in the grinding cracks and resulte in final cracking of the mould.