The fracture cause of the aluminium motor wheel was analyzed by means of chemical composition analysis,metallography microstructure examination and fracture analysis.The results show that some defects such as metallic inclusion,unmerited heat treatment process and so on,conduce the existence of bulky acicular silicon and herringbone(α+Si) two-phase eutectic structure,which reduces the mechanical properties.They cause the brittle fracture of aluminium motor wheel.