Ge preferential adsorption on Si(111)7×7 surface at the initial stage has been investigated by ultra-high vacuum scanning tunneling microscopy (UHV-STM). We demonstrate that there is a critical nucleus for the adsorbed Ge clusters on Si(111) 7×7 surface. The center sites of the Ge clusters are located in the areas encircled by three adatoms. Moreover, on the Ge clusters the local density of states near the Fermi level is drastically reduced, compared with that far from the Fermi level.