Abstract: The grain boundary parallel to the edge of the “island-shaped zone” (ISZ) near the carbon steel fusion line of the dissimilar steel joint was studied. It was shown that the grain boundary parallel to the edge of the ISZ might be the migrated grain boundary or the solidified grain boundary. The ISZ had three kinds of structures, the first was consisted of the carbon steel base material and the composition transition region of its periphery, and the elements content in the composition transition region was between the carbon steel base material and the weld metal; the second was only consisted of the composition transition region, and the elements content in the composition transition region was also between the carbon steel base material and the weld metal; the third was only consisted of the composition transition region, but the Ni content in the composition transition region was higher than 5% − 6%. Only the ISZ with the first and second structures were possible to form the migrated grain boundary parallel to the edge of the ISZ, but the ISZ with the third structure could not form the migrated grain boundary parallel to the edge of the ISZ. The solidified grain boundary parallel to the edge of the ISZ was formed by the intersection of short cellular grains growing perpendicular to the ISZ surface with their adjacent subgrain bundles.