Abstract: Based on irreversible thermodynamics, the damage evolutionequation is constructed by introducing the damage driving force, in order toreduce the work of the up-down method in identifying the fatiguelimit. The theoretical relationship between fatigue life and stress orstrain can be obtained by integrating the damage evolutionequation. For smooth members of structure, according to the results offatigue experiment with group method, the material parameters of damageevolution equation, including the fatigue limit, are determined by the leastsquares technique. The curves of fatigue life in high stress area areobtained by the damage mechanics. And then, the fatigue life and fatiguelimit in low stress area are deduced. The results of theoreticalfatigue limit agree well with the fatigue experimental results ofup-down method. By using this method, a lot of time, cost and manpowerin fatigue experiment can be saved.