The effect of short-time duplex heat treatment on the microstructure and mechanical properties of Ti-6Al-4V alloy was investigated. This heat treatment consisted of solution treatment at 1203 K for 60 s and subsequent water-quenching plus aging at 753, 853 and 953 K for 40 s. The yield strength and tensile strength of the alloy were significantly increased by the above heat treatment and their maximum improvement rates reached about 25%. It was thought that this strengthening was caused by the formation of 0 martensite phase with quenching after the short-time solution treatment and the precipitation of fine phase in the retained phase during the short-time aging. In spite of the remarkable improvement in the strength, the ductility of the heat-treated materials remained above the level of the non-treated material, most likely because strain-induced martensite transformation occurred in phase which was retained even after the short-time aging.