Abstract: Based on the mechanical properties of twenty- sevenalloy steels and aluminum alloys, the theoretical fatigue limits are analyzed with three methods. It is found that in the three methods only twocan give a proper estimate of the fatigue limits. Moreover, when therelationship among the strength coefficient, the strain-hardeningexponent and the mechanical properties is considered, the two methodscan be used to estimate the fatigue limits from the conventional mechanicalproperties. The limitation of highstrength steel in the estimation may be lifted.