A quantitative model of unstable fatigue-crack growth is proposed on the basis of a determination of the specific plastic strain energy stored the distance X* from the crack tip. The model makes allowance for the energy irreversibly dissibated within the damage zone and the resistance of the material to brittle fracture during static loading. The model makes it possible to predict the transition from stable to unstable crack growth, the length of the brittle crack jump, and the number of load cycles and length of stable crack growth between brittle jumps. The proposed model is checked using steel 15Kh2MFA at 293 K (σ0.2 = 954 MPa). The theoretical and experimental data on the laws of unstable fatigue-crack growth is shown to agree satisfactorily.