The microstructure evolution of a new titanium alloy Ti-29Nb-13Ta-4.6Zr prepared by vacuum casting was observed with aging treatment at 450 ℃. The results reveal that the microstructure in the alloy exhibits single under-cooled metastable β phase after solid solution treatment, while the mixed structure can be created in the alloy, which is composed of stable β phase dispersively distributed on αphase in grain and strip α phase as well as stable β phase adjacent to strip α phase in boundary, due to the transformation of metastable β phase into strip α phase during aging treatment.