The morphology and microstructures of Ti-6.5Al-2Zr-1Mo-1V (TA15) samples at different stages of martensite decomposition fabricated by selective laser melting are characterized during the in situ tensile test, and the deformation and crack mechanisms are discussed. The results show that the as-deposited samples with full martensite structure, former being recognized as a state of poor plasticity, possess excellent ductility (with a maximum displacement of 0.74 mm) due to the cooperative deformation behavior of hierarchical martensites. The full-annealed sample, consisting of lamella (α + β) laths, also shows good ductility (0.65 mm) with the enhanced collaborative deformation behaviors between α and β laths. However, for samples with partially decomposed martensite structure, abrupt fracture after yielding was observed with deteriorated ductility (0.54 mm), as the slipping of dislocations is hampered by the dispersive nano-β precipitations. [ABSTRACT FROM AUTHOR]