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The effect of tensile and torsional prestrains on fatigue lives of 304 stainless steels is compared. The fatigue life as a function of prestrain amplitudes exhibits an ‘S’ curve, and the inflexion point of the S curve is affected by prestrain modes and cyclic strain amplitudes. An interesting phenomenon is that when the onset stress of secondary hardening reaches 300 MPa, intergranular martensite begins to be formed. Combining martensite distribution, fatigue life and cyclic stress responses, the onset stress of secondary hardening is proposed to reflect the location of martensite nucleation sites. Compared with tensile prestrain, torsional prestrain results in higher stress. In addition, higher prestrain and cyclic amplitudes also lead to higher cyclic stress and hence earlier nucleation of intergranular martensite. Furthermore, the influence mechanism of prestrain modes and cyclic strain amplitudes on the inflexion point of S curve is revealed.