Abstract: General mathematical expressions for S-N curve were introduced. The most used power function Sm·N=C expression was transformed into the standard power law expression S=C1/m·N -1/m. Fatigue data for three materials of weld joints were processed according to the expression and standard power function relation S=C·Nn. The results were compared and it manifested that different disposing methods would result in completely different S-N expression. Standard deviation was bigger but conditional fatigue limit was smaller derived from the former power law expression than the latter,which would provide reference for fatigue design to select S-N expression in engineering application.