Corrosion fatigue tests were conducted in synthetic sea water to investigate the influence of flow rate on crack growth behavior under various corrosion potentials. The effect of increasing flow rate under a free corrosion potential was to increase the cathodic reaction rate of dissolved oxygen reduction. This results in increase of crack propagation rate. When the potential was controlled electrochemically at minus 450 mv (vs. Ag/AgCl), it showed different mechanisms with respect to flow rates. The impressed current density at a low flow rate was high enough to maintain a high crack propagation rate.