The low cycle fatigue tests of SA508 Gr.1a low alloy steel were carried out to investigate the fatigue crack growth mechanisms in high temperature water. The fatigue life in 310oC deoxygenated water was shorter than that in air. Furthermore, the reduction in the fatigue life in 310oC deoxygenated water was enhanced with a decreasing strain rate, from 0.4 to 0.008 %/s. The ductile striations with the streamed down features, which may indicate the occurrence of the metal dissolution, were mainly observed at the strain rate of 0.008 %/s. And the flat facets and the brittle cracks, which may be evidences for the HIC, were primarily observed in the strain rate range from 0.04 to 0.4 %/s. From the analysis of microstructure, it is thought that the HIC contribute dominantly to the reduction in the fatigue life in the strain rate range from 0.04 to 0.4 %/s and the metal dissolution is mainly responsible for the reduction in the fatigue life at the strain rate of 0.008 %/s.