The present study is concerned with the effects of plate thickness on high-cycle fatigue properties and fatigue crack propagation behavior of investment cast Ti-6Al-4V alloys having Widmanstratten structure. High-cycle fatigue test and fatigue crack propagation test were conducted on three cast plates having different thickness, and then the test data were analyzed in relation with microstructures, tensile properties, and fatigue fracture mode. The high-cycle fatigue results indicated that fatigue strength of the three cast plates was quite similar because of their similar tensile strength. In the case of the fatigue crack propagation, the thicker cast plate composed of thinner a platelets had the slightly faster crack propagation rate than the other plates. The effective microstructural factor determining the fatigue crack propagation rate was found to be the thickness of a platelets because it was well matched with the reversed cyclic plastic zone size calculated in the threshold DeltaK regime. (C) 2004 Kluwer Academic Publishers.