The fatigue tests of SA508Cl.3 were performed to characterize the behaviors of crack growth rate in 288℃ water with various loading frequencies. The crack growth rate was increased by decreasing the frequency until a critical frequency in high dissolved oxygen(DO) condition. Fractographic studies showed that the enhancement of crack growth rate was environmentally assisted by the hydrogen embrittlement, since brittle striations and cleavage like facets with microvoids observed on the fracture surface. The strain rate effects on crack growth rate was investigated through da/dt vs. dε/dt curves. At intermediate range, there was a transient point corresponding to an onset of dynamic strain aging, where an abrupt increase in the crack growth rate was observed. Above the transient point, small size particle enhanced brittle cracks while before the transient only large size particle enhanced brittle cracks. From the observations of the sectioned area of the specimen, microcracks were found to be resulted from strain localization along slip bands in the crack tip yielding zone. In addition, there was a ductility loss in tensile test with hydrogen charged specimen at 288℃. From these results, it is suggested that EAC may be enhanced by a specific strain rate and that EAC may be related to interactions of hydrogen with the oxide film and to Luders band movement with a high strain gradient at inclusion/matrix interface.