The atomic arrangements of the α-7×7 phase and incommensurate phases of Al adsorbed Si(111) surface are investigated respectively by scanning tunneling microscopy (STM). STM images of α-7×7 surface reveal that characteristic triangular structures consisting of three Al adatoms situated on the center part of each half unit cell. The three Al adatoms form a triangular cluster and are bonded to the center adatoms. In this situation, the dangling bond of the center adatom is saturated, which would modify its electronic structure into non-metallic. The incommensurate phase is consists of approximately 9×9 structures, which are separated each other with misfit dislocations. In the “9×9” structure, individual Al atom is visible arranged in threefold symmetry.