Deformation and fracture behavior of Ti-55531 alloy with lamellar microstructure (LM) and bimodal microstructure (BM) under static loading were analyzed by the in-situ SEM tensile method. The results show that the characteristic parameters of the alpha phase have a strong influence on the deformation, crack initiation and propagation behavior of the alloy under static loading. Dislocation initiated at secondary alpha (α_s) interior under pressure, because the thick α_s plate is softer than residual beta (β_r) lath in LM. Dislocation moves and accumulates at the α_s/β_r interphase during deformation, which induces that cracks initiate at and propagate along the α_s/β_r interphase during the process of fracture. However, beta transaction microstructure (β_trans) is greatly harder than equiaxed primary α (α_p) in BM, moreover, α_pwith large size could support longer slip length. Both of above reasons leads to a large number of multi-direction slips initiate at α_p during deformation of BM. The intersection of slip lines in different directions promotes stress concentration at α_pinner, and some dislocations accumulate at the α_p/β_trans interface to produce stress concentration. These behavior results in the initiation of microcracks within α_p and the α_p/β_trans interface, and propagation along the weak interface of α_p/β_trans, or the aggregation site of α_p.