Strengthening by grain refinement and increasing dislocation density through highpressure torsion (HPT), which is an attractive technique to fabricate ultrafine grained and nanostructured metallic materials, is expected to provide β-type Ti-29Nb-13Ta-4.6Zr (TNTZ) higher mechanical strength while maintaining low Young's modulus because they keep the original β phase. However, the ductility shows reverse trend. Greater strength with enhanced ductility can be achieved by controlling precipitated phases through HPT processing after aging treatment. Aged TNTZ subjected to HPT processing at high N exhibits a homogeneous microstructure with ultrafine elongated grains having a high dislocation density and consequently non-equilibrium boundaries and distorted subgrains with non-uniform shapes and nanostructured intergranular precipitates of α phases. Therefore, the effect of HPT processing on the microstructure and mechanical hardness of TNTZ after aging treatment was systematically investigated in this study. TNTZ, which was subjected to aging treatment at 723 K for 259.2 ks in vacuum followed by water quenching, subjected to HPT processing at rotation numbers (N) of 1 to 20 under a pressure of around 1.25 GPa at room temperature. The microstructure of TNTZAT consisted of precipitated needle-like α phases in β grains. However, TNTZ AHPT at N ≤ 10 comprises very fine α and small amount ω phases in ultrafine β grains. Furthermore, the hardness of every TNTZAHPT was totally much greater than that of TNTZAT. The hardness increased from the center to peripheral region of TNTZAHPT. In addition, the tensile strength of every TNTZAHPT was greater than that of TNTZAT. The tensile strength of TNTZAHPT increased, but the elongation decreased with increasing N and then both of them saturated at N ≤ 10.