While the precipitation of α (hcp) from β (bcc) in various Ti alloys has been investigated in great detail, less is known about the precipitation of β from α. This study conducts a systematic investigation of structures and migrations of interfaces between β precipitates and α′ matrix in a Ti-2.6 wt% Mo alloy. Utilizing conventional and highresolution transmission electron microscopy (TEM), assisted with molecular dynamics and generalized O-element analysis approaches, we unveil the dislocation and atomic structures of three typical portions of the interface surrounding a β precipitate, i.e., the habit plane, the side facet, and the end face. The habit plane of β precipitates contains two sets of dislocations with Burgers vectors b$_1$ = [$2\overline{11}3$]$_α$ / 6 |[$1\overline1 1$]$_β$ / 2 and b$_4$ = [$2\overline{113}$]$_α$ / 6|[$11\overline1$]$_β$ /2, in contrast to the single set of b1 dislocations previously found on the habit plane of α laths precipitated from β matrix in various Ti alloys. Interface migrations during shrinkage of β precipitates are characterized via in-situ TEM, showing that both the habit plane and side facet migrate through the lateral motion of nanometer-high growth ledges. The dislocation structures of various growth ledges were obtained through generalized O-element calculations, suggesting a potential nonconservative dislocation trajectory as semicoherent growth ledges sweep across the interface.