In order to overcome the embrittlement of metastable titanium alloys caused by the precipitation of ω_iso phase during aging, regulation of isothermal ω precipitation was investigated in Ti−15Mo alloy. The results show that the sample is brittle when direct aging (A) is applied at 350 °C for 1 h after solution treatment (ST). If pre-deformation (D) is performed on the ST sample to induce {332} twins and secondary α′′ phase, subsequent aging at 350 °C (STDA350) improves the strength to 931 MPa with a good ductility of about 20% maintained. However, when aging is performed at 400 °C or 450 °C (STDA400/450), the strength can be further improved, but the ductility is dramatically reduced. Atomic-scale characterizations show that the partial collapse of ω phase in the STDA350 sample effectively eliminates aging-induced embrittlement, but complete collapse leads to poor ductility in the STDA400/450 sample.