Titanium alloys have been increasingly used for downhole tubulars and components for corrosive oil wells due to their combination of strength, density, and corrosion resistance. The present study investigated the effect of annealing temperature on microstructures and mechanical properties of a Ti-6Al-4V-0.5Ni-0.5Nb alloy developed for oil well application. The α/β- and β-rolled Ti-6Al-4V-0.5Ni-0.5Nb alloy plates were subjected to annealing at different temperatures ranging from 750 to 900 ℃ and hold for 1 h, and the microstructure and mechanical properties were evaluated. The experimental results show that the developed alloy exhibited both high strength and ductility than that of the conventional Ti-6Al-4V alloy. The α/β- and β-rolled alloy after annealing exhibited bimodal structures and provided well-balanced strength, ductility and impact toughness, high strength of 950−1000 MPa, elongation of 17%−18%, and impact toughness of 90−120 J/cm2. From mechanical viewpoint, the new alloy after appropriate annealing is suitable for oil well applications.