Ti-6Al-4V specimens were fabricated by selective laser melting (SLM) to study the effect of thermal treatment on the phase transformation, elemental diffusion, microstructure, and mechanical properties. The results show that vanadium enriches around the boundary of α phases with increasing annealing temperature to 973 K, and α′ phases transform into α+β at 973 K. The typical α′ martensite microstructure transforms to fine-scale equiaxed microstructure at 973 K and the equiaxed microstructure significantly coarsens with increasing annealing temperature to 1273 K. The SLM Ti-6Al-4V alloy annealed at 973 K exhibits a well-balanced combination of strength and ductility ((1305±25) MPa and (37±3) %, respectively).