The effect of cathodic protection on corrosion fatigue crack growth behavior of mild steel in natural sea water was investigated by measuring the rate of fatigue crack growth. Assuming that the threshold stress intensity factor range ΔKth corresponds to the crack growth rate at (1-2)×10-8mm/cycle, ΔKth was 3.9MN/m3/2 in air, 2.3MN/m3/2 in sea water, and 5.4MN/m3/2 under the cathodically polarized condition of -800mV (S. C. E.) in sea water respectively. The correlation between the stress intensity factor range and the rate of crack growth follows Paris's law at the growth rate from 3×10-7 to 2×10-6mm/cycle in air and under -800mV (S. C. E.) in sea water, and at the growth rate from 1×10-7 to 1×10-6mm/cycle in sea water. The rate of crack growth in sea water was found to be twice faster at the maximum than that in air, but its rate under -800mV (S. C. E.) in sea water decreased to approximately 40% of that in air within these ranges of growth rate. Scanning electron microscopy was employed to study the fractography of corrosion fatigue crack surface of specimens. The calcareous deposits were found to form within the crack surfaces of the specimen cathodically polarized in sea water. This retardation of the corrosion fatigue crack growth in the specimen cathodically protected in sea water can be explained by the wedge action of calcareous cathodic reaction products from sea water which were found within the crack.