<p>This study comparatively investigates the fatigue crack initiation and propagation mechanisms of a novel air-hardening steel with a thin film-like martensite-austenite (M-A) island and bainitic ferrite microstructure and a conventional air-hardening steel with a ferrite-martensite microstructure. The results show that fatigue cracks in the conventional air-hardening steel initiate from a single site on the surface. In contrast, the novel air-hardening steel also exhibits single-site surface crack initiation under low stress levels, whereas simultaneous crack initiation at both surface and interior sites is observed at high-stress levels. The internal crack initiation is mainly attributed to local stress concentration induced by a high density of dislocation tangles at bainitic ferrite lath boundaries. During the fatigue crack propagation stage, the novel air-hardening steel exhibits superior resistance to crack growth. This enhanced crack growth resistance is likely related to the higher density of Σ3 coincident site lattice boundaries in the bainitic ferrite microstructure, which effectively impede crack propagation. In addition, the formation of abundant shear bands and secondary cracks within the bainitic ferrite absorbs part of the energy required for main crack propagation, thereby further reducing the crack growth rate.</p>