During Selective Laser Melting (SLM) process melted metal rapidly cool down generating ultrafine microstructures and high residual stresses inside the material. The purpose of this study was to investigate the effects of two post-heat treatments on the microstructure and mechanical properties of the A357 alloy samples fabricated via SLM: a stress relieving heat treatment, and a T6 heat treatment. The first treatment was performed at 300 °C for 2 hours to release the thermal stresses in order to produce parts without distortion when detached from the building platform. Such treatment produces a reduction of hardness and tensile strength and an enhancement of ductility as compared to the as-built condition. Subsequently, a T6 treatment was performed consisting of a solution treatment for 8 hours at 540 °C, water quenching and following ageing treatment at 170 °C for 3 hours. This T6 treatment increases again the yield strength and elastic modulus to values comparable to the as-built conditions, leading to a coarser and more isotropic microstructure.