The aim of this study is to compare the corrosion resistance of X37CrMoV5-l tool steel after nanostructurization and after a conventional heat treatment. The nanostructuring treatment consisted of austempering at 300°C, which produced a microstructure composed of nanometric carbide-free bainite separated by nanometric layers of retained austenite. The retained austenite occurred also in form of blocks which partially undergo martensitic transformation during final cooling. For comparison, a series of steel samples were subjected to a standard quenching and high tempering treatment, which produced a microstructure of tempered martensite. The obtained results showed that the corrosion resistance of steel after both variants of heat treatment is similar. The results indicate that the nanocrystalline structure with high density of intercrystalline boundaries do not deteriorate the corrosion resistance of steel, which depends to a greater extent on its phase composition.