Abstract <p>Dual-phase titanium alloys are widely employed as ultrasonic waveguide materials for medical applications. However, the mechanisms underlying the interaction between microstructure and ultrasonic propagation in the alloy remain unclear. Herein, variations in the ultrasonic attenuation behavior of medical extra-low-interstitial-grade Ti6Al4V alloys with microstructure evolution are systematically investigated. Experimental results demonstrate that within a <i>d</i> (grain size)/<i>λ</i> (wavelength) range of 0.1–1, scattering attenuation and dislocation damping are the key factors affecting ultrasonic attenuation. Longitudinal-wave scattering attenuation is directly proportional to the grain size, and dislocation damping is primarily influenced by the orientation of α grains. In particular, dislocation slip on the prismatic planes of α grains is found to be a key factor influencing the dislocation-damping effect for dual-phase Ti alloys. This can be attributed to the oscillatory movement of dislocations in response to ultrasonic-wave disturbance when the wave propagation direction is parallel to the prismatic planes of α grains, causing energy dissipation. Further, the mechanisms by which dislocation types and movement patterns influence ultrasonic attenuation are discussed in detail based on a G–L dislocation pinning model.</p> Graphical Abstract <p></p>