Dynamical observation of Ga adatoms on GaAs(001) surfaces was successfully made by a system using scanning tunneling microscopy(STM) and molecular beam epitaxy(MBE), which were not separated into each chamber but equipped as an incorporate unit. It is found that Ga adatoms were self-organized to a unit cell far from the B-step edge and on the missing dimer row of (2×4) β 2 As-adsorbed surface. Moreover, the three Ga adatoms formed a trimer cluster which changed to a tetramer cluster with addition of one Ga atom. Ga heptamers were also observed. The dangling bonds of a Ga dimer were completely empty, satisfying electron counting heuristics. No clear observation by the filled state imaging indicates that the adatoms are crystalline. However, the Ga trimer clearly observed is metal cluster. The Ga trimer and heptamer have 9 and 21 electrons in the molecular orbital states. The numbers are closed to the magic numbers of spherical jellium, 8 and 20, respectively. This excess electron might become the support between the cluster and the substrate surface, forming 2D magic clusters, the Ga trimer and heptamer, on GaAs(001) surface.