In the present paper fracture appearance and metallogra-phical structure of aluminum alloy die castings weighing from 0.24 to 4.8Kg were examined during in probing into factors affecting the properties of load-subjected die castings.The existance of broken chilling layer was found in those castings. The" forming mechanisms of broken chilling laver and its influence on the propertiesof die castings were studied. The work has a practical significance to the improvemend of quality of die casting and to the prevention of rejects caused by tke broken chilling layer.