The effect of the fatigue size of a Ti4822 alloy under typical service temperatures (650 ℃)was investigated through high-temperature ultrasonic fatigue experiments on three types of samples with different parallel segment diameters (2, 3, and 5 mm). A method for predicting the high-temperature very high cycle fatigue (VHCF) limit of Ti4822 alloy was proposed on the basis of the highly stressed volume theory. The crack initiation mechanisms of typical fatigue samples were also analysed. The results indicate that the high-temperature VHCF limit of Ti4822 is influenced by the size effect, with the diameter of the parallel segment of the sample increasing, leading to a decrease in the VHCF limit. The proposed method for predicting the high-temperature VHCF limit of Ti4822 alloys could predict the fatigue limit within the fitting range and

might be applicable for predicting the VHCF limit of Ti4822 alloys at other temperatures. The crack initiation mechanics of the Ti4822 alloy include two types: nonmetallic inclusion slip initiation and grain boundary slip initiation, and the crack initiation location moves from the surface toward the internal region as the stress decreases.