Abstract Fully lamellar Ti-6Al-4V alloys comprise body-centered cubic (BCC) β lamellae in large-sized, hexagonal close-packed (HCP) α colonies and exhibit outstanding toughness. Although α/β interfaces are considered to play a key role in plastic deformation connected to the toughness, the interface effects have not been revealed so far. In this work, we studied underlying deformation mechanisms of interface-related deformation modes at an atomic scale. After the cyclic loading, { 1 1 ¯ 02 } deformation twins were observed in the vicinity of fatigue crack surfaces. Moreover, the α/β interface structures before and after cyclic loading deformation were characterized via transmission electron microscopy (TEM). The initial α/β interfaces can be described by the terrace ledge kink model, consisting of ( 01 1 ¯ 0 ) α|| ( 1 ¯ 21 ) β terrace plane and ( 1 ¯ 100 ) α|| ( 1 ¯ 01 ) β ledge plane. TEM investigations reveal that deformation twins nucleate at the α/β interface and the corresponding nucleation is ascribed to the dissociation of basal type dislocations. More importantly, these twins can continuously propagate through multiple β phase lamella. The continuous propagation of twinning is accomplished through double kinking mechanism. In this manner, twinning in α phases and sequential kinking in β phases can effectively release the stress intensification at the crack tip and dissipate plastic work/energy, correspondingly enhancing fracture toughness of fully lamellar Ti-6Al-4V.