Abstract: The microstructure of Ti-6 Al-4 V alloy fabricated by selective laser melting was studied at different heat treatment conditions to reveal the internal relation during the heat treatment process,microstructure and mechanical properties.The results show that at 600～700 ℃ α′ martensite in the alloy is not completely decomposed to form a mixed microstructure of α′ and α+β,resulting in a high strength and low elongation for the alloy.When the temperature increases to 800～900 ℃,α′ martensite in the alloy can be fully transformed into steady lamellar α+β.After hot isostatic pressure,the metastable α′ martensite is completely converted to lamellar α+β and some local areas in the alloy are coarsened.With increase of the heat treatment temperature,the yield strength and tensile strength of the alloy are gradually decreased coupled with gradual increase of the specific elongation,compared with the deposition state.The optimum annealing temperature and holding time of the as-fabricated samples are determined at 800 ℃ for 2 h in combination with the growth behavior of grains.