In this work, the differences in the microstructure and mechanical properties of the Ti modified Al-Cu alloy along the selective laser melting (SLM) build direction are investigated. The results showed the mechanical properties decrease gradually from bottom to top along the building direction, the hardness decreases from 121HV at the bottom to 108HV at the top, the tensile strength also decreases from 375.8MPa to 359.2MPa, while the ductility is not improved. Transmission electron microscopy observation confirms that numerous θ' phase is formed in the bottom, while rarely in the top, indicating that the difference of mechanical properties is caused by the θ' phase, which is formed during SLM due to the substrate heating. Additionally, the top sample requires longer time to reach peak hardness during post-heat treatment at 200°C. Therefore, appropriate substrate temperature and post-heat treatment are necessary for age-hardenable alloys to achieve consistent mechanical properties along the building direction.