An oriented head-to-tail cluster formation of an endohedral yttrium metallofullerene, Y @ C-82, On a Cu(111)1 x 1 clean surface has been observed directly by the use of a scanning tunneling microscopy (STM) under ultrahigh-vacuum conditions. The STM observation reveals that the Y @ C-82 molecules preferentially form dimers and one-dimensional clusters at the step edge of the Cu(111) surface. The observed intermolecular distance (11.2 +/- 0.5 Angstrom) is smaller than that of the simple Y @ C-82- - -Y @ C-82 van der Waals distance (11.4 Angstrom), indicating the presence of a strong interfullerene interaction. The STM results suggest the presence of strong dipole-dipole and charge-transfer interactions among Y @ C-82 fullerenes. Such a molecule has a great similarity to the "superatom" features proposed theoretically in a semiconductor heterostructure.