Abstract: Many mechanisms of the formation of spheroidal graphite in ductile iron are no general agreement at present. Surface adsorption theory,defect growth theory and star-shaped model have lager effects and higher recognition. However,circumferential growth model and Double′s graphene mechanism are scarcely discussed at home. The fundamental divergence between those spheroidization mechanisms is whether the preferential c-axis growth is a necessity. Basing on that,the author classify them into two types in this article,and then particularly and carefully examine and evaluate referring to why the spherulitic shape should be the nature form of graphite and the effect mechanisms of S,O,Mg and other elements as well as the evolution process of graphite. The Double′s theory is considered to be a more reliable spheroidization mechanism. The star-shaped model is more complicated,although it can account for the formation of explode graphite rather well,this is not sufficient for it to being a reliable spheroidization mechanism because the key evidences are controversial.