Effects of α phase nucleating at a transition phase and dislocation on mechanical properties in a metastable β titanium alloy, Ti-6.8Mo-4.5Fe-1.5Al(mass%), were investigated. Tensile test and optical, scanning, and transmission electron microscopy were used. Two-step aging promoted the precipitation of a ne α phase in grain interiors because the α phase nucleated at a transition phase of β ′ . A precipitation-free zone appeared at rst, but nally disappeared during aging. A specimen that included the precipitation-free zone showed lower yield strength and larger elongation than it did without the precipitation-free zone. This was because dislocation could easily glide in the precipitation-free zone, which was veri ed from examination of the intergranular ductile fracture surface. The transition phase of β ′ promoted the disappearance of a precipitation-free zone, due to the promotion of α phase precipitation, resulting in enhancement of yield strength and deterioration of elongation. It is likely that the width of PFZ and the crystallographic orientation of β grains determine fracture mode to be intergranular ductile, intergranular brittle or intragranular fractures. Aging after tensile deformation produced α phase precipitation at dislocation. A speci c variant of α phase was selected due to a stress eld around the dislocation that included a planar slip band. This resulted in the formation of a large colony of the speci c α variant, while elongation deteriorated because the large colony probably facilitated crack propagation.