The precipitation strengthening and tensile properties in ultrasonic-affected friction stir welding (UFSW) were investigated using Al-Cu alloy and compared with another joint produced by the same setup but without applying the ultrasonic. Both the nucleation rate and growth rate of theta' phase during UFSW were significantly promoted by the ultrasonic. More importantly, the precipitate distribution was extremely homogeneous in the UFSW. The ultrasonic inducing excess vacancies via reducing the vacancy formation energy was experimentally confirmed by positron annihilation spectroscopy and molecular dynamics simulation. The vacancy formation energies with and without the effect of the ultrasonic at 300 K were 0.60 +/- 0.01 eV and 0.67 +/- 0.01 eV, respectively. A quantitative estimation suggested that the vacancy concentrations during UFSW and CFSW were about 3 x 10(-4) and 5 x 10(-5), respectively, indicating that it was increased by almost one order of magnitude due to the application of the ultrasonic. On this basis, a vacancy-induced theta' precipitation mechanism was proposed. Considerable Cu-vacancy pairs due to the high-density vacancies induced by the ultrasonic contributed to copious nucleation sites for theta' phases. The vacancy migration and condensation onto the pre-existing phase interface can relieve the coherency strains, permitting a rapidly unrestricted growth of the precipitates. Finally, simultaneous enhancement in tensile strength and elongation was confirmed by applying the ultrasonic.