The precipitation and coarsening behavior of θ′ phase in Al-3.5Cu and Al-3.5Cu-0.5In alloys (mass fraction, %) were investigated by transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and some other methods. In addition markedly accelerates the aging response of Al-Cu alloy aged at 175 oC, and raises the peak hardness by about 200 MPa. TEM observation shows that In-rich particles precipitate uniformly at the early stage of aging, which will act as heterogeneous nucleation positions of θ′ phase; In′ is observed at a corner of platelets having their broad face in the plane of view; θ′ phase is coarsened much more slowly in In-added alloy than in In-free alloy. With 6% pre-deformation before aging, these two alloys have similar aging response rate and strength, which confirms that in T6 state Indium facilitates θ′ precipitation mainly through quenched-in vacancies.