Abstract: The mean fatigue life curves of a materialcan be obtained from damage mechanics. The materialparameters that affect the S-N curve and the relationship betweenthem are obtained. Themean fatigue life curves, namely, the material parameters corresponding toK_t=1.0, are obtained by using the up-down method and the groupfatigue test. One needsonly to use the up-down fatigue test to get the fatigue limit underany coefficient of stress concentration K_t, then, we can get the meanfatigue life curves corresponding to the stressconcentration.