A thermomechanical process was optimized to improve fatigue strength while maintaining a low Young's modulus in a biomedical β-type titanium alloy, Ti-29mass%Nb-13mass%Ta-4.6mass%Zr (TNTZ) alloy. For this purpose, short-time aging combined with severe cold rolling was employed in this study. The TNTZ alloy was aged at 573 K for various time periods after cold rolling. As a result, the ω phase was observed in the TNTZ alloys subjected to the above-mentioned process. Among these TNTZ alloys, the samples aged for 3.6 ks and 10.8 ks showed a good balance between tensile strength and elongation. However, in the case of the 3.6 ks samples, the tensile strength was improved, but the fatigue strength was not improved remarkably. On the other hand, in the case of the 10.8 ks samples, the tensile strength and the fatigue strength were both improved by this process. This fatigue strength is the highest among the TNTZ alloys with a relatively low Young's modulus (below 80 GPa).