Abstract <p>Crystallization of cast iron is a nanostructured process in which the main role is played by nanocrystals of iron, graphite and silicon. Dendritic microcrystals of austenite are not solid solutions of carbon and silicon atoms in the γ–Fe crystal lattice. During crystallization of cast iron, hydrogen and oxygen atoms in their melts serve as demodifying elements of the eutectic austenite structure. Hydrogen, oxygen and sulfur atoms of cast iron melts during crystallization as demodifying elements reduce the dispersion of eutectic graphite microcrystals. Oxygen and sulfur atoms of liquid cast iron during crystallization prevent branching of dendritic microcrystals of eutectic graphite. Molecular hydrogen released on dendritic microcrystals of eutectic austenite prevents their branching, but promotes branching of dendritic microcrystals of eutectic graphite. Modifying elements of cast iron refine the structure of the austenite-graphite eutectic and promote the formation of branched dendritic microcrystals of eutectic graphite. Magnesium has the strongest modifying effect.</p>