This study investigates the influence of ultrasonic nanocrystal surface modification (UNSM), a novel surface treatment technology, on the mechanical characteristics and rotary bending fatigue performance of solution-treated and annealed (STA) Ti-6Al-4V and Ti-6AI-4V extra low interstitial (ELI) alloys. Various techniques have been developed to use surface modification in order to improve fatigue performance of machine parts. The UNSM treatment emerges due to its ability to induce a distinctive combination of modifications in the top surface and the subsurface of the material. The mechanical properties are acquired and compared for UNSM-treated and untreated specimens. Fatigue data is acquired and compared for UNSM-treated and untreated specimens of Ti-6Al-4V ELI alloy by subjecting specimens to rotary-bending fatigue tests. Fracture analysis of failed specimens is performed. The improvement in the fatigue performance is observed for UNSM treated specimens. This improvement is attributed mainly to the UNSM-induced combination of following modifications: compressive residual stress, micro-hardness, and nanograin in the subsurface of the material.