This study conducts failure tests using a simulated specimen to investigate the effect of thermal aging on the deformation and failure behaviors of system, structure, and components (SSCs) of nuclear power plants (NPPs) made of cast austenitic stainless steels (CASSs) under excessive seismic loads. Both unaged and thermally aged CF8A CASSs were used for the experiment, and the large cyclic loads in the form of displacement-control and load-control were applied at a quasi-static displacement rate. Displacement-controlled tests were performed at room temperature (RT) and 316oC and load-controlled tests were performed at RT. The results show that the deformation behaviors of aged CF8A CASS under both types of cyclic load are almost the same as those of unaged CF8A CASS. The thermal aging slightly promotes the failure of CF8A CASS under displacement-controlled cyclic loads, but the failure of specimen still occurs under the cyclic load levels several times higher than the load of the design basis earthquake. Under load-controlled cyclic loads, thermal aging retards the failure of CF8A CASS. Consequently, the thermal aging has no apparent negative effect on the deformation and failure behaviors of CASSs under large cyclic loads, even if it considerably changes the strength, ductility, and fracture toughness of CASSs.