Fatigue crack growth rate tests conducted in water at room temperature and 300 °C on SS 304L(N) material. The dissolved oxygen levels in water were varied from 10 ppb to 1500 ppb. Higher fatigue crack growth rate in water at room temperature under 0.005 Hz is primarily due to deformation induced martensite. Higher fatigue crack growth rate in water at 300 °C is due to corrosion-fatigue synergistic damage.