The fatigue crack growth behavior of the bimodal Ti-6Al-4V alloys with two different volume fractions of the primary α phase (α p ) of 76 and 36% is investigated by the in situ testing technique. The experimental results show that the crack growth rate of the α p ¼ 36% Ti-6Al-4V alloy is lower than that of the α p ¼ 76% one. The local fatigue crack growth rate is evidently decreased by the various boundaries including α p grain boundaries, boundaries between the α p phase and basketweave microstructure, and α/β lamellar interfaces. A criterion associated with the boundary characteristics is obtained to evaluate the grain boundary resistance to the fatigue crack growth in the engineering alloys.