A Zr-Ta-Nb alloy made of biocompatible alloy elements only and consisted of single β-phase with bcc structure was prepared by conventional solidification. Electron diffraction, transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM) were used to examine microstructural evolutions of the β-Zr-Ta-Nb alloy subjected to different heat treatments. The results show that the metastable β-phase in the as-solidified Zr-Ta-Nb alloy turned to decompose when annealed at 400°C and this led to the formation of a highly strained structure containing nanosized precipitates and local disordered regions. Striking phase separation including the formation of the equilibrium α-phase was found to occur at 600°C. By means of nanosized electron-probe analysis of energy dispersive X-ray spectroscopy, the compositions for the nanosized phases formed due to different heat treatments were analysis.