The effect of cooling rate on microstructure and tensile properties of BT25y titanium alloy were studied using OM, SEM, XRD, and TEM methods. The results indicate that the microstructure of the air-cooled specimen consists of α and β transformed phase, while in oil- and water-quenched microstructure, α, α" and α" martensite phase were found. Lattice parameters of α" phase obtained by XRD analysis show that a is shorter, b is longer in water-quenched microstructure than in oil-quenched one. After aging treatment at 550 ℃/6 h followed by air cooling, the phase component is the same with its unaged counterpart in air-cooled condition; martensite phase decomposed into stable α and β phase in oil- and water-quenched conditions. Room temperature tensile properties after aging treatment were tested. Compared with the air-cooled sample, tensile and yield strength of the sample after oil and water quenching were much higher, accompanied with much lower elongation. The phenomenon was thought to be related to the shape and size of secondary α phase.