The influence of microstructures (β grain size, α/β colony size and width of α lath) and stress ratios (R=0.1, 0.7) on fatigue crack growth (FCG) behavior of beta-annealed Ti624x (x=2, 4 and 6) alloys was investigated. The FCG rate curves, crack paths, and the plastic zone ahead of the crack tip were compared. The reducing FCG resistance from Ti6242 to Ti6246 was found to stem from decreasing α/β colony size. Increasing stress ratio also reduced crack deflection and crack closure, and thus decreased the FCG resistance. The relevant mechanisms were discussed in terms of Kmax and crack closure effect.