There is provided a method of manufacturing a stack package. The method includes vertically stacking core dies on a base die wafer to provide a stack structure, forming partition walls on the base die wafer to surround the stack structure, and forming an underfill material layer that includes under-filling portions filling gaps between the core dies, and filling fillet portions covering side surfaces of the core dies. The fillet portions are formed to have a width confined by the partition walls. The partition walls are removed, and a mold layer is formed to cover the fillet portions. Related stack packages are also provided.