The superplastic tensile behaviors of Ti-6Al-4V (TC4) titanium alloy were tested at temperature ranges from 850 ℃ to 950 ℃ and tensile velocities in the range of 0.3–6.0 mm/min. The microstructures of the specimen under various deformation parameters were evaluated by scanning electron microscope (SEM) and transmission electron micrograph (TEM). The results indicate that the maximum tensile force decreases with the increase of temperature and the decrease of tensile velocity. The optimum tensile elongation of 1120.5 % is attained at 900 ℃ and 0.6 mm/min. α and β phase grain sizes increase with the increase of temperature and the decrease of tensile velocity. β phase volume fraction plays an important role in superplasticity, and the best elongation is attained under the condition of ~7.0 µm α phase and ~4.0 µm β phase and β phase volume fraction is about 31 %. High deformation velocity leads to finer grains and results in larger tensile force.