Investigated was the structural stability of the cast heat-resistant alloy 45Х26НЗЗС2Б2 at the temperature of 1150°C. Significant qualitative and quantitative changes in all structural components and the individual phases of the alloy in the range of exposure time to 100 hours were found. It was detected that the highest intensity of diffusion processes in the alloy is observed in the initial period of exposure up to ~ 2 h.