The phase transformations during thermome-chanical processing can be employed to optimize mechanical properties of β-type Ti alloys.However,such understandings are still lacking for the alloy consisting of dual β+α＂ phases in solution-treated and quenched state.In this paper,the phase transformations in a Ti38Nb model alloy subjected to different thermomechanical processing were investigated by using synchrotron X-ray diffraction(SXRD)experiments,and their influence on the Young\'s modulus was discussed.The results indicated that high-density dislocations introduced by cold rolling still existed after annealing at temperatures lower than 573 K,which can decrease the martensitic transformation start tempera-ture to below room temperature.With annealing tempera-tures increasing,the α\"→ β,β → ωiso,and β → α phase transformations occurred successively.At annealing tem-perature of 473 K,the specimen consisted of a trace of α＂and ω phases as well as dominant β phase which was kept to room temperature by the high density of dislocations,rather than by the chemical stabilization.As a result,an ultralow Young\'s modulus of 25.9 GPa was realized.Our investigation not only provides in-depth understandings of the phase transformations during thermomechanical processing of β-type Ti alloys,but also sheds light on designing biomedical Ti alloys with ultralow Young\'s modulus.