In situ X-ray synchrotron microtomography was used to quantify the evolution of damage in the titanium alloy Ti5321 with bimodal microstructure. The results show that voids nucleate in the equiaxed primary α grains and α plates or at α/β interfaces. There exists an approximate linear relationship between volume of voids and local strain of several largest voids. The low growth rate and delay of coalescence of voids are attributed to the dispersed distribution of the α phase in the bimodal microstructure. There are no micro-cracks formed in the interior of the sample until fracture. The deep dimples in the fracture surface indicate ductile fracture. The damage mechanism of Ti5321 is discussed with the help of post-mortem SEM. To clarify the difference in damage mechanism between metastable β and α + β titanium alloys, Ti64 is also in situ tensile tested.