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The morphological evolution of ErSi 2 nanowires on Si(001) with Er coverage was investigated by scanning tunnelling microscopy, low-energy electron diffraction and x-ray diffraction. At low Er coverages, elongated ErSi 2 nanowires in a hexagonal crystal structure are formed. The nanowires are exclusively oriented perpendicular to the dimer rows of the Si(001) substrate, thus suggesting a method to control the nanowire arrangement via the substrate configuration. At 3 ML of Er rectangular islands with flat tops are observed, which crystallize in the tetragonal lattice of ErSi 2 . Our experiments suggest that the structural transformation from hexagonal to tetragonal takes place when the nanostructures reach the height of the c-axis of the tetragonal unit cell.