The topic of this final thesis is the influence of heat treatment on the mechanical properties of ductile cast iron. The paper describes the most commonly used heat treatments that are conducted on ductile casting, but the focus was on isothermal heat treatment. In the theoretical part of the thesis, the general properties of ductile iron and the influence of isothermal heat treatment on the mechanical properties of ductile iron are stated. In the practical part of the thesis, two preliminary experiments were carried out to control the available ductile iron, which determined that it was unalloyed ductile iron EN-GJS-400. On the basis of the determined material, heat treatment austempering (isothermal improvement) was chosen with the aim of achieving a bainitic structure. The test plan was generated using a central composite design. After heat treatment, the hardness of the test tubes was measured, and statistical processing of the data was performed. The obtained results were presented and commented in the paper.