Wire arc additive manufacturing (WAAM) is being considered as a technology to replace the conventional manufacturing process of titanium alloys. However, coarse β grains, which can extend through several deposited materials, result in strong textures and anisotropy. As a solution, we study the plastic deformation effects of ultrasonic needle peening (UNP) on the microstructure. UNP treated materials deform plastically and the dislocation density increases. Fine α+α' grains with low aspect ratio are observed in the UNP treated specimens. UNP treated WAAM Ti-6Al-4V alloys have higher strength and lower elongation than those characteristics of WAAM Ti-6Al-4V alloys. Due to UNP treatment, the z-axis directional specimens exhibit a greater effect of reducing elongation than do the x-axis directional specimens. The UNP treatment produces fine grains in proportion to the number of times UNP is performed, thereby increasing strength. UNP processes produce a large number of dislocations in the WAAM Ti-6Al-4V alloys, with the most dislocations being formed at the surface.