Abstract: Hard spots were found in the die cast cylinder block of ADC12 aluminum alloy. The knife phenomenon appeared during machining and the product formed the micro cracks. The causes of hard spots in ADC12 aluminum alloy die casting cylinder block were analyzed by methods of hardness testing, composition analysis, metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that the hard spots formed after the impurities enter into the die casting cylinder block of the aluminum alloy, cause by low purity of nitrogen, aluminum water, and not slagging in time. Therefore, the knife phenomenon appeared during machining. Finally, the corresponding improvement measures are presented.