A novel β-type titanium alloy Ti–29Nb–13Ta–4.6Zr (TNTZ) has been developed and extensively researched to achieve highly desirable mechanical properties such as a high strength while maintaining a low Young’s modulus that is close to that of bone, as an alternative candidate for conventional titanium metallic biomaterials such as Ti-6Al-4V ELI. Therefore, strengthening by grain refinement and increasing dislocation density is expected to provide TNTZ high mechanical strength while keeping a low Young’s modulus because they keep the original β phase. In this case, severe plastic deformation, such as high-pressure torsion (HPT) processing, is a potential treatment for obtaining these properties. Furthermore, HPT processing is effective for producing ultrafine-grained TNTZ having high dislocation density in single β structure. The obtained promising results, which are a tensile strength of around 1100 MPa and a Young's modulus of around 60 GPa, motivated that the above mentioned mechanical properties can be achieved by microstructural refinement through HPT processing However, the mechanism of microstructural refinement is unclear for TNTZ during HPT processing. Therefore, the aim of this study is to investigate microstructural changes of TNTZ through HPT processing by X-ray diffraction analysis and transmission electron microscopy. The microstructures of TNTZ subjected to cold rolling (TNTZCR) and HPT processing (TNTZHPT) comprised single β grains; however, the intense β {110} peak reveals that the preferred orientation is β <110> for TNTZHPT. While the microstructure of TNTZCR shows a comparatively high dislocation density (2.3 x 1016 m -2 ), HPT processing leads to a drastic accumulation of dislocations (5.3 x 1016 m -2 in dislocation density). Dislocations in the microstructures of TNTZHPT are well arranged for the cell wall and/or subgrain boundaries, with a stronger dipole character than random distribution. The dislocation density, arrangement parameter and crystallite diameter (around 11 nm) of TNTZHPT saturate from the center to the peripheral region of a coin shaped specimen at N = 20. Therefore, such a microstructural saturation in a specific strain inducing, N = 20, suggests a threshold for further investigation of β-type titanium alloys.