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.