Abstract: There is a greater eutectic graphitizing expansion of D. C. I when it freezes. As the expansion is suppressed by the casting mold wall, dislocations are led to produce in the matrix due to the deformation of Austenite, When the rigidity of the casting mold is sufficiently high, the expansion is suppressed to the extent that the morphology of the dislocations in the matrix is in the form of cells, such change of substructure of matrix has been proved by the examination of the samples using a TEM. Provided the dislocation cells once appear in the matrix, the combined mechanical properties of the metal are bound to increase. This phenomenon is theoretically and practically illustrated in this paper and a direction of further research and development of due-tile iron is pointed out here by the author.