Precipitation response of a recently developed b-type Ti–25Nb–2Mo–4Sn alloy(wt%) during aging was investigated in detail. Experimental results indicate that the metastable ω or equilibrium α phase can form alternatively even under the same aging condition, depending on the condition prior to the aging, i.e., solution treatment or severe cold rolling. This provides, for the first time, the direct evidence for the competition between ω and α in aged β-type Ti alloys. This peculiar aging response is found to be closely related to high-density dislocations and grain boundaries which suppress the formation of ω but favor the precipitation of fine a phase. Precipitation response of a recently developed b-type Ti-25Nb-2Mo-4Sn alloy (wt%) during aging was investigated in detail. Experimental results indicate that the metastable ω or equilibrium α phase can form alternatively even under the same aging condition, depending This provides for the first time, the direct evidence for the competition between ω and α in aged β-type Ti alloys. This peculiar aging response is found to be closely related to high-density dislocations and grain boundaries which suppress the formation of ω but favor the precipitation of fine a phase.