The Al-Si-Cu-Mg cast aluminum alloys have high mechanical properties and good cast performance. Due to their excellent comprehensive properties, the Al-Si-Cu-Mg cast aluminum alloys have wide application, and have become one of the most important structural materials applied in the equipment manufacturing industry. Actually, many key components in practical engineering application are often subjected to the alternating load, and thus the fatigue failure has become an important factor which concerns the safety and economy for those structures used in various engineering fields. Although some researches for the fatigue behavior of aluminum alloys have been performed, mainly focus on the regularity understanding. Especially, the influences of rare earth elements and heat-treat condition on the low-cycle fatigue behavior of aluminum alloys have not been comprehensively revealed. Obviously, the investigation concerning the microstructure and fatigue property of the Al-Si-Cu-Mg cast aluminum alloys can not only provide the theoretical basis for the development of new type cast aluminum alloys but also the reliable theoretical foundation for the safety design and reasonable use of these alloys. In order to determine the influence of rare earth element Sc on the low-cycle fatigue behavior of casting Al-9.0%Si-4.0%Cu-0.4%Mg alloy with T6 treated state, the cyclic stress response behavior, fatigue life behavior and cyclic deformation mechanism of the Al-9.0%Si-4.0%Cu-0.4%Mg(-0.3%Sc) cast aluminum alloys with T6 treated states under low-cycle fatigue loading condition were investigated. The results show that at the low total strain amplitude, the Al-9.0%Si-4.0%Cu-0.4%Mg alloy exhibits the cyclic strain hardening during whole fatigue deformation, while the Al-9.0%Si-4.0%Cu-0.4%Mg-0.3%Sc alloys exhibit the cyclic strain hardening in the initial stage of fatigue deformation and then the stable cyclic stress response in the later stage of fatigue deformation. At the higher total strain amplitudes, the Al-9.0%Si-4.0%Cu-0.4%Mg(-0.3%Sc) alloys exhibit the cyclic strain hardening. The addition of Sc can effectively enhance the cyclic deformation resistance and prolong the fatigue lives of the Al-9.0%Si-4.0%Cu-0.4%Mg alloy with T6 treated state. At the lower total strain amplitudes, the cyclic deformation mechanism of the Al-9.0%Si-4.0%Cu-0.4%Mg(-0.3%Sc) alloys with T6 treated state is the plane slip, while at the higher total strain amplitudes, the cyclic deformation mechanism becomes the wavy slip.