Electro- and thermomigration of metallic islands of mu m size formed by vapor deposition on the Si(100)2*1 surface have been investigated using an ultrahigh-vacuum scanning electron microscope (UHV-SEM) at substrate temperature higher than the melting point of the islands. The direction of electromigration versus the electric current depends on the type of metal, whereas the direction of thermomigration is always from the cold to hot side independent of the type of metal. The velocity of islands is approximately proportional to the island radius and increases exponentially with temperature for both cases. The activation energies of electro- and thermomigration are approximately 0.6 eV for Au-Si islands having a eutectic composition. The driving forces which act on the islands are discussed based on the diffusion theories for electro- and thermomigration.