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The tensile deformation and fracture behavior of a particular semi-equiaxed microstructure (S-EM) in a near alpha titanium alloy TA19 are investigated by an in situ method. In the S-EM, the thin β lamellae grow through the equiaxed α_p phase (α_p), and the original α_p/β_trans interface in the bimodal microstructure largely disappears, forming a blurry interface between the semi-equiaxed α_p phase (equiaxed α_p phase that is grew through by the thin β lamellae) and the transformed β microstructure (β_trans). The formation of dense slip bands inside the semi-equiaxed α_p phase in the S-EM is inhibited by the thin β lamellae during the tensile deformation. The special characteristics of the S-EM reduce the stress concentration at the interface, and the crack initiation probability in the blurry semi-α_p/β_trans interface decreased compared to the distinct α_p/β_trans interface in a conventional equiaxed microstructure (EM). Moreover, the ultimate tensile strength of the S-EM is higher than that of the EM with a slight loss of plasticity.