Abstract: The serrated grain boundaries can promote the creep property and retard the corrosion cracking of material. Therefore, we apply magnetic field in the process of semi-solid heat treatment to form the shape-controllable micron-scale serrated grain boundaries without pre-deformation or high-temperature in-situ deformation. Experimental results indicate that without pre-deformation nor high-temperature in-situ deformation, the application of an external magnetic field successfully enables the formation of shape-controllable micron-scale serrated grain boundaries. As the external magnetic induction intensity increases, the proportion of serrated grain boundaries progressively increases to 61.06%. Under the influence of the external magnetic field, the formation of serrated grain boundaries is closely related to the dislocation-assisted grain boundary migration and quasi-liquid film migration. The transformation of grain boundary morphology makes the development of aluminum-based materials with excellent mechanical properties possible.