The u′ precipitation process of Al Cu alloy was simulated with the coupling model of a phase field and a concentration field during isothermal annealing and under thermal irradiation. This model successfully described the u′ precipitation process under isothermal annealing at 423 K and 473 K, including the growth rate of the u′ precipitates. The model was used also to simulate the u′ precipitation process under thermal irradiation at 473 K and it was found that the size and the distribution of the u′ precipitates can be controlled locally by this process. The simulation results also indicated that the heat sink process around the irradiated area is important to achieve a precisely controlled precipitated structure.