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A refined fully lamellar microstructure with a prior ¢ grain size of ³100 µm was obtained for a 0.1 mass percent (%) B-modified Ti6Al 4V alloy. On the other hand, there were no morphological differences in equiaxed microstructures between the B-free and B-modified alloys; both had ¡ grain sizes of about 8 µm.The very high-cycle fatigue (VHCF) lifetimes in the regime from 10E+6 to 10E+10 cycles were measured for these microstructures by using hourglass-shaped specimens and an ultrasonic fatigue test machine at a frequency of 20 kHz, while high-cycle fatigue (HCF) lifetimes in the regime from 10E+4 to 10E+7 cycles were measured using smooth cylindrical specimens and a hydraulic servo fatigue test machine at 10 Hz and a fatigue ratio R of 0.1. The VHCF and HCF behaviors of B-modified alloys were found to be highly dependent on both microstructures and the level of applied stress. In the VHCF regime, where applied stress is well below the conventional fatigue threshold, the B-free and B-modified alloys exhibited the same fatigue strength, in other words, the same fatigue lifetime within the same microstructure. A set of fatigue lifetime data for B-free and B-modified alloys with equiaxed microstructures indicated that these alloys had higher fatigue strength than the alloys with lamellar microstructures in the VHCF diagram in the whole cycle range of up to 10E+10 cycles. The overall trend in the HCF lifetime or strength was that the addition of 0.1 mass% B had either a favorable effect or no influence on the fatigue life, depending on the level of applied stress. Above a certain level, which corresponded to a cycle regime up to about 10E+6 cycles for a lamellar microstructure and up to about 10E+7 cycles for an equiaxed microstructure, the fatigue lifetime was remarkably prolonged by the addition of 0.1 mass% B. However, in the regime beyond these cycles, when the stress level decreased, the favorable effect of B was gradually diminished and the HCF lifetime of the B-free and 0.1 mass% B-modified alloys were coincident in both lamellar and equiaxed microstructures; this coincidence was maintained thereafter. The mechanism for the dependence of the VHCF and HCF strength of the B-modified alloy on microstructures and the level of applied stress is proposed and discussed. [