Abstract: The fatigue crack propagation rate test was conducted on 45 steel after quenching at different temperatures (760,800,840℃) and tempering at 350℃, and then the fatigue crack propagation rate curves were obtained. The effects of quenching temperature on the fatigue crack propagation behavior were also studied. The results show that compared to that quenched at 840℃ (conversional quenching), the undissolved ferrite existed in the microstructure of tested steel after subcritical quenching at 760 and 800℃. After subcritical quenching at 800℃, the martensite structure was fine and the ductility was good. The fatigue crack propagation rate curves of tested steel were in accordance with Paris law and the crack propagation rate decreased with the increase of quenching temperature. The fatigue fracture was composed of crack low propagation region, crack stable propagation region and final fracture region. After subcritcal quenching at 800℃, the low propagation region was relatively flat; the stable propagation region was relatively rough and showed quasi-cleavage morphology; the final fracture region showed obvious tearing edge, shear lip and dimple morphology, with the height difference of about 283.9 μm, indicating a large proportion of dutile fracture.