In order to obtain good mechanical properties of ductile cast iron, it is necessary that the highest quantity of the regular graphite spheroids are distributed uniformly in a matrix. The paper presents the method that employs the specific criterial quantity - casting modulus for calculating the carbon equivalent, total carbon content and effective wall thickness of a casting. The effective wall thickness is also adjusted by the correction coefficient χ expressing the influence of the thermo-physical properties of the moulding sand on the solidification time. The present range of ferrous based materials used for castings the volume of special grades of cast irons increases, when the production of the steel castings decreases permanently. From a long-term point of view, the ductile cast iron, known as the spheroidal graphite iron (SGI), shows sustained steady growth of this production. The graphite takes the shape of nodules due to the conditions of solidification produced by the active elements introduced into the molten metal during its metallurgical treatment. The basic nodulizing element Mg added in amounts from 0.03 - 0.06% with traces of Ce, Ca etc… causes the tendency of cast iron to solidify metastably, and therefore, the inoculation has to be done simultaneously or as soon as possible. The aim of inoculation is to form the graphite's nuclei of the supercritical size and promote their growth. To attain good mechanical properties of individual grades of cast iron it is necessary to give increase the regular nodules of the graphite as much as possible and distribute them uniformly throughout the matrix. This depends not only on the chemical composition of the molten metal, but also on the cooling gradients given by the casting section's thickness and the heat conductivity of the mould. 2 DETERMINATION OF THE CHEMICAL COMPOSITION OF DUCTILE CAST IRON The effect of the casting's section on the structure and mechanical properties of non- alloyed SGI was examined on casting samples with a step-plate section. The thicknesses of steps were 3, 6, 12, 24, 48, and 96 mm, and their width and length were always six times the corresponding thickness. The meltings were performed in an induction furnaces with the crucible of 100 or 500 kg using the charge based on basic pig iron having an S content of 0.016 - 0.020%. The materials of the individual melting had an equivalent carbon content (CE) of 3.9 to 5%, when the C content was in the range of 3 to 4.2% and the Si content from 1.9 to 3.5%. To ensure the reproducibility of the results for the metallurgical processing of molten iron, the base iron was poured simultaneously over the nodulizer and the inoculant in the ladle was used and, for comparison, we apply the in-mould technology. For treatment of the molten iron, the modifier of