Spherical aberration-corrected high-resolution transmission electron microscopy images reveal that Ti in the carbon-nanotube-reinforced-Ti (CNT/Ti) composites synthesized via the high-pressure torsion method has a face-centered cubic (fcc) lattice structure. The intrinsic properties of the fcc/hexagonal close-packed (hcp)-Ti boundary with orientation relationships of [111⎯⎯] fcc//[0002]hcp and {011}fcc//{21⎯⎯1⎯⎯0} hcp between the fcc-Ti and hcp-Ti are revealed through a first-principles investigation. Interface relaxation verifies that the nucleation of the fcc-Ti structure requires a minimum stable thickness of three atomic layers. When the pressure increases, the interface adhesion work, W ad, of the fcc-Ti(111⎯⎯)/hcp-Ti(0002) interface decreases, the surface energy increases, and the interface bonding strength of the fcc/hcp-Ti boundary decreases. Moreover, the hcp-Ti(0002)/fcc-Ti(111−)boundary becomes unstable under high pressure, enhancing the nucleation and growth of the fcc-Ti phase at the boundary. The results of this study help in understanding the correlation between the inherent attributes of the fcc/hcp-Ti boundary, and the properties of CNT titanium composites are improved.