Ultra-long cycle fatigue fracture origins in high strength steels are mostly at non-metallic inclusions due to the influence of the trapped hydrogen. In the vicinity of a non-metallic inclusion at the fracture origin, an optically dark area (ODA) is often observed inside a fish-eye mark, which represents the particular morphology associated with mechanism of failure at an early stage: hydrogen assisted fatigue. From an analysis of several features involved in this mechanism of failure, as the development of the ODA, the threshold for pure fatigue propagation, as a function of crack length and inclusion size, an expression was deduced to estimate the internal fatigue limit, that is, the stress level below which fracture produced by cracks initiated from an internal inclusion is not found after 10 10 cycles: σ e Int =256 ΔK th R i max where Δ K th is the pure fatigue crack propagation threshold as a function of crack length estimated with the following expression: ΔK th =4×10 −3(H V +120)(3R i max ) 1/3 If ΔK th ≤10 MPa m 1/2 Otherwise ΔK th =10 MPa m 1/2 R i max is the maximum radius of the non-metallic inclusions from which the crack initiates, in μm, and H V is the Vickers hardness, in kgf mm −2. The internal fatigue limit was estimated for several steels and compared with experimental results obtained from the literature for internal fatigue lives in the range of 10 8–10 10 cycles. Differences between the estimated internal fatigue limit and the experimental one ranged from 4 to −29%.