The paper presents the results of a study of the possibility of using lanthanum hexaboride nanoparticles as a modifier of high-chromium austenite and martensitic steels for the purpose of obtaining the finer cast structure and changing the nature of the nonmetallic phase. The samples were melted in a Tamann furnace. Optical microscopy and X-ray microspectral analysis were used to study the chemical composition, macroand microstructure of the obtained ingots, and to analyze non-metallic inclusions. It was found that the introduction of nanoparticles of lanthanum hexaboride leads to obtaining of the finer dendritic structure of steels of both classes. The addition of lanthanum hexaboride can be an efficient way of introducing boron into high-chromium steels, including nitrogen-containing ones. It was shown, that the introduction of boron in the form of lanthanum hexaboride prevents the formation of tertiary boron nitrides located along the boundaries of the dendrites.