Fatigue crack growth behavior at low loading frequencies for a Mn-Ni-Cr steel immersed in 3.5 % NaCl solution, with and without cathodic polarization, is investigated and presented in this paper. Frequency shedding method is used to estimate fatigue crack growth rate over a range of frequencies between 0.01 and 5 Hz at a constant stress intensity factor range of 15MPa Hm. The effectiveness of cathodic potential of - 900 mV SCE in containing corrosion contribution to crack growth is estimated by comparing with published data on fatigue crack growth rate in lab air and 3.5 % NaCl solution without cathodic polarization. It is noted that there are three regions of crack growth rate as a function of applied frequencies when the data is plotted in log-log scale: linear crack growth rate, plateau crack growth rate, and high crack growth rate. The crack growth rate of steel in 3.5 % NaCl solution at 15MPa Hm is scanned for different cathodic potentials between - 760 mV SCE and 1150 mV SCE at 0.01 and 0.1 Hz. The potentials at which the crack growth rates are the maximum and the minimum are found to be - 760 and - 950 mV, respectively. The optimum cathodic protection potential for the minimum corrosion fatigue crack growth rate ranges between - 900 and - 950 mV. This optimum potential for the minimum corrosion crack growth rate is more negative than the cathodic potential required for restraining the corrosion effects on fatigue strength of steel to normal ambient air behavior. © 2014 by ASTM International.