The endurance limit of polycrystalline pure iron is high. To understand its mechanism, tensile strength (σB), cyclic yield stress (σyc), and fatigue limit (σw) were evaluated for both single-crystalline and polycrystalline pure iron. The endurance ratio (σw/σB) of rotary bending test and axial loading using polycrystalline specimens ranged from 0.58–0.68. Moreover, the σw/σB of axial loading using a single-crystalline specimen indicated a high value of 1.06. Even without considering the strain rate dependence, the σw/σB of pure iron was higher than that of general-purpose steels such as low-carbon steel, general structural steel, stainless steel, and martensitic steel. Contrary to these results, σw/σyc of single-crystalline and polycrystalline pure iron showed the same value (0.881) as that of general-purpose steels. Thus, σyc would be more correlated with σw than σB. Furthermore, attempts were made to quantify the effect of strain rate, residual stresses, and work-hardened layers on fatigue limit. Consequently, the change in the endurance ratio (0.58–0.68) was quantitatively explained by the strain rate, residual stress, and work-hardened layer.
