Ti-6Al-4V parts obtained by selective laser melting typically have an acicular α' martensitic microstructure whose ductility is low. Thus, post-heat treatments are useful for increasing ductility. In this work, the effects of sub-β-. transus heat treatments on the mechanical properties of Ti-6Al-4V parts with porous structures are correlated with martensite α' phase decomposition. The precipitation of β phase and the gradual transformation of α' into α phase by the diffusion of excess vanadium from α' to β phase are proposed to be the main events of martensite α' phase decomposition in parts fabricated by selective laser melting. The heat treatment performed at 650. °C for 1. h produced no microstructural changes, but the samples treated for at the same temperature 2. h showed a fine precipitation of β phase along the α' needle boundaries. The heat treatment performed at 800. °C for 1 or 2. h produced a fine α+β microstructure, in which β phase are present as particles fewer in number and larger in size, when compared with the ones present in the sample heat-treated at 650. °C for 2. h. Heat-treatment of the parts at 800. °C for 2. h proved to be the best condition, which improved the ductility of the samples while only slightly reducing their strength.