The hardness and thermal stability are the important problems of the cast Al–Cu alloy related to the microstructural changes. In order to increase the possibilities of high temperature applications of the cast Al–Cu alloy, it is necessary to gain a more detail understanding of the correlation between the age hardening and the microstructure in the cast Al–Cu alloy, and the thermal stability of the θ′ precipitates at elevated temperatures. The aim of this work is to investigate the effects of La addition on the age hardening behavior and precipitation kinetics in the Al–Cu alloy in the temperature range from 435 to 523 K. The results indicated that La addition considerably increases the number of the θ′ precipitates and decreases their sizes, which results in the enhanced age hardening effect. The precipitation kinetics analysis showed that the activation energy (13 kJ/mol) of the θ′ precipitate nucleation of the modified alloy is smaller than that (19 kJ/mol) of the unmodified alloy. The decrease in the activation energy of the θ′ precipitate nucleation can be explained with both the enhanced nucleation process due to La/Cu/vacancy aggregating and the increased interaction between Al and Cu atoms.