Abstract: As to the welded joints made of A537、A131、E36、15MnNi and other steels the fatigue testing has been executed in sea water and atmosphere. By using with the double-logarithmic coordinate system of log da/dN and log ΔK or log da/dN and log ΔJ the experimental data have been fitted with a straight line or two lines, thus the fitting equations da/dN=C(ΔK)^m and da/dN=A(ΔJ)ⁿ(or da/dN=B(ΔJe)^β and da/dN=D(ΔJ_p)^γ)have been obtained. Having evaluated the fit curve by the F test and the others, the statistical results show that the fitting is relatively accurdte, so these equations are feasible and predical.