A. A. Lebedev, I. V. Makovetskii, UDC 620.178.15/179.119 N. R. Muzyka, and V. P. Shvets The paper presents results of the experimental investigation of deformation behavior and damage accumulation kinetics in a heat-resistant reactor pressure vessel steel at high stress levels under stress-controlled (pulsating), low-cycle loading up to 10 4 cycles. Hardness measurements have been performed upon various operating time periods under elastoplastic deformation. It has been found that hardness and strength of the steel exhibit qualitatively different behaviors throughout the lifetime. The damage accumulation process in steels under static and cyclic loadings has been studied by means of the LM-hardness method. The curves of Weibull homogeneity coefficient vs. cycling stress and accumulated strains are obtained.