To investigate the influence of different modification gradient fields on the fatigue crack initiation mechanisms of the Ti6Al4V alloy, this study employed two surface treatments, polishing (P) and polishing combined with ultrasonic rolling process (P+USRP), to generate distinct modification gradient fields for the examination of crack initiation mechanisms, as illustrated in the figure. Display omitted •In our investigation, we employed polishing (P) and polishing combined with ultrasonic rolling process (P + USRP) as surface strengthening treatments for the Ti6Al4V alloy, leading to different modification gradient fields.•The specimens with different modification gradient fields underwent high-cycle fatigue tests, showing different levels of enhancement in the alloy’s fatigue performance.•The differences in modification gradient fields affected the fatigue crack initiation mechanisms of the Ti6Al4V alloy. In this paper, the high-cycle fatigue (HCF) properties and microcrack initiation mechanism of Ti6Al4V alloy after polishing (P) and after polishing followed by the ultrasonic surface rolling process (P + USRP) treatments are investigated. The results indicate that variations in modified gradient fields would alter the crack initiation location and mode. Specifically, after P treatment, the fatigue crack initiation source occurs approximately 205 μm from the surface, owing to localized strain formation within the internal αp/αp grain boundaries under cyclic loading, thereby promoting rapid cleavage at the (0001) twist interface and evolving into microcracks. This result aligns with the crack initiation mechanism of an untreated specimen. Conversely, fatigue crack initiation after P + USRP treatment occurs approximately 425 μm from the surface, as a result of the deeper modified gradient field, where the crack initiation mechanism results from the competing effects of rapid cleavage at the (0001) twist interface of αp/αp and the accumulation of numerous microvoids. The difference in strain between the α and β phases causes the α/β interface to become a weak region of dislocation accumulation, leading to the formation of microvoids induced by microvoid aggregation. Both P and P + USRP are beneficial for improving the fatigue performance of the alloy.