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A new metastable β titanium alloy, Ti-25Nb-10Ta-1Zr-0.2Fe, was designed for biomedical applications. The alloy ingot was prepared by vacuum consumable electrode arc melting. The solid-solution aging behavior of the alloy was investigated. The phase composition and the microstructure were studied by XRD, TEM and SEM. The tensile strength after aging and the hardness for different aging time were tested. Results show that only the α″ phase is found after solid solution treatment at 800 oC for 1 h. The ω phase and β phase are observed when the alloy is subjected to aging treatment at 400 oC for 0.5 h. The ω phase, as a brittle phase, keeps a high volume fraction during the aging treatment, which leads to a sharp increase of the Vicker hardness. In this case, the tensile strength of the alloy decreases dramatically to only about 400 MPa, which is much smaller than that of the specimen in the cold-rolled condition (about 750 MPa). Hence, a large mount of ω phase decreases the tensile property of the newly designed alloy Ti-25Nb-10Ta-1Zr-0.2Fe