Systematic variations of the exposure time and point distance of the pulsed laser used in selective laser melting (SLM) of Ti-6Al-4V resulted in three representative microstructures: the fully martensitic α', the near-α' containing a small amount of isolated β, and the fully lamellar α/β. The energy density in SLM determines the steady-state temperature in a deposited layer reached by balance between heat input from the subsequent layers and heat loss into the previous ones. A critical energy density was revealed below which no in-situ α' decomposition occurred. On the other hand, the in-situ formation of fully lamellar α/β was obtained using energy density higher than this critical value, leading to a steady-state temperature above that for α' decomposition for a sufficiently long duration. All three microstructures exhibited high tensile yield strength of 1100–1150 MPa, with excellent tensile elongation in the fully martensitic α' (~15%) and fully lamellar α/β (~12%) but significantly lower ductility in the near-α' (< 6%). The best ductility observed in the fully martensitic alloy dismisses the myth that α' is inherently brittle. Instead, the brittleness in the near-α' alloy can be attributed to the presence of thin β lamellae on the primary α' plates oriented at ~45° to the tensile axis, causing substantial stress concentration at the α/β interface.