The relationship between crack sensitivity and mechanical and thermal properties at high temperature on crack formation in the aluminum. alloys casting has been investigated. In this study, a series of experiments on deformation behavior of 6xxx and 7xxx alloys in the solid-liquid coexistence region have been made. The tensile strength and ductility have been measured by using a high temperature tensile testing machine which can melt in a portion of the sample and solidify as the requested cooling rate with an electromagnetic induction heating coil. And, the thermal properties at high-temperature region of each alloy were also measured. By comparing the breaking section of the tensile testing sample with breaking one of DC billet, the formation of hot tear and cold cracking was clarified.