This paper is concerned with the investigation of the effect of magnesium to sulfur ratio on the graphite morphology and estimation of the ranges of this ratio use to produce graphite cast iron at different section sections. The main factors affecting shape of graphite cast iron are the metallurgical structures and the section thicknesses. Cast iron of different shapes of graphite particles directly affect its thermo-mechanical properties. The nodular shape of these graphite particles such as in ductile cast iron improve its mechanical properties, on the other hand, when the shape of these graphite particles become elongated such as in flake graphite cast iron results in improving its thermal conductivity. In between, the worm-like shape of these graphite particles such as in compacted graphite cast iron, make this type of cast iron to have thermo-mechanical properties in between those of ductile and flake graphite cast iron. The different types of ductile, compacted and flake graphite cast iron were produced by means of plunger method at different section thicknesses and the effect of Mg/S ratio on these types of graphite particles was investigated and its range was established.