The results are presented of an investigation of the cyclic crack resistance of 15Kh2MFA steel (σ0.2=583.5 MPa, σt = 700 MPa) in a broad range of change in specimen thickness (t=25–150 mm) and conditions of single-frequency (R=0.1–0.75) and double-frequency (q=0.005–0.75, p=1000) cyclic loading. It is established that the combined influence of specimen dimensions and the investigated cyclic loading conditions on the rules of fatigue crack development may appear in the whole range of change of stress intensity factors, from Kth to Kfc, and has a complex character, the specific appearance of which is determined by specimen thickness, cyclic loading parameters, and the effective stress intensity factor.