The microstructure evolution and tensile properties in a metastable β titanium alloy, Ti–5Al–5Mo–5V–3Cr-0.6Fe alloy with different aging conditions were investigated. The microstructure evolution of ω and α phase precipitated during low-temperature aging was characterized by the dilatometer test and transmission electron microscopy. The results show that the nanoscale ω phase formed during low-temperature aging cannot affect the tensile properties due to its small size which fails to change the deformation mechanism or strengthen the β phase. Meanwhile, the small size ω phase also fails to assist secondary α (α_s) nucleation and refine α_s during high-temperature aging due to ω dissolution. The α phase precipitated preferentially during low-temperature aging plays a considerable role in refining lamellar α_s by occupying the space of β matrix that hinder the growth of lamellar α_s. The α+β solution-treatment with two-step aging successfully refines the lamellar α_s which in turn increased the tensile strength with small reduction of ductility.