The precipitation of secondary phase and phase transformation behavior of a solution-treated Ti-6Al-4V alloy during aging heat treatment are investigated. The content, size of the equiaxed α phase, and the thickness of secondary phase are greatly influenced by the aging temperature. With the increased aging temperature, the mean grain size of the equiaxed α phase significantly decreases, whereas the precipitation process of the secondary phase is accelerated. Furthermore, during aging heat treatment, the precipitation and growth of the secondary phase not only destroy the initial lamellar α phase, but also connect with the equiaxed α phase to form a new curved lamellar α phase. In addition, it is found that the α 0 phase is precipitated, and the α 0 !α þ β phase transition occurs with the increased aging temperature.