Implanting a spinal fixture by using implant rods is one of the effective treatments for spinal diseases. Most of the implant rods are made of the Ti-6A1-4V ELI alloy (Ti64). However, some problems regarding using Ti64 rods have been pointed out; these rods contain vanadium, which is highly toxic to the human body, and exhibit a considerably higher Young's modulus than the cortical bone. The Ti-29Nb-13Ta-4.6Zr alloy (TNTZ) developed by the authors exhibits good biocompatibility due to its nontoxic- and nonallergic-components. Further, TNTZ has a lower Young's modulus as compared to Ti64. It is expected that implant rods made of TNTZ exhibiting low Young's modulus will prevent the accelerated degeneration of the segment adjacent to a fusion. The mechanical properties of TNTZ rods subjected various heat treatments are comprehensively evaluated through tensile tests, Young's modulus measurement, and four-point bending fatigue tests based on the ASTM F2193 standard and compared with those of Ti64 rods. Among the TNTZ rods used in this study, the TNTZ rod aged at 673 K (Rod673K) exhibits the highest tensile strength because of the precipitation of the isothermal ω-phase and a-phase in it. The tensile strength of Rod673 K exceeds that of Ti64, while the increase in Young's modulus of the former is suppressed below that of the latter. With regards to four-point bending fatigue strength, no advantages are obtained from every TNTZ rods in comparison with Ti64 rod.