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Tensile fracture of near-β-titanium alloys with bimodal microstructure （BM） is very sensitive to micro-void nucleation. In this study, micro-void nucleation, growth, and propagation during tensile fracture of Ti-55531 alloy are investigated. α/β grain boundary (α/β-GB) and α/β grain boundary shear (α/β-GBSH) micro-voids are the main void sources, accounting for 83% of the total voids. The reason for the formation of α/β-GB voids is the stress concentration caused by the difference in the strength of the primary α (αp) and β transformation (βtrans) microstructure and the low strength of the non-coherent αp/β interface. α/β-GB voids eventually lead to fractures along the αp/β interface. α/β-GBSH voids are caused by the difference in the strength of the αp phase, the grain boundary α phase and the βtrans structure, which will cause the αp phase to tear and fracture along the grain boundary α-phase. The {112¯2} twin is first observed in near-β-titanium alloy, which explains the high plasticity of the alloy.