A Si microstructure on a Si (111) 7 x 7 surface was investigated by a noncontact atomic force microscopy (ncAFM) and a scanning Kelvin probe microscopy (SKPM) in ultra high vacuum. The Si microstructure was generated by intermittent contact of a cantilever tip. It was found by the ncAFM and SKPM observations that the Si mound with a height of 1 Angstrom and a width of 30 nm was generated, and the surface potential of the small mound was 0.1 V lower than that of the 7 x 7 domain. A quenched Si (111) surface was also observed by the ncAFM and SKPM. The differences in height and potential between the reconstructed 7 x 7 and the disordered 1 x 1 domains were about 1 Angstrom and 0.1 V, respectively. Therefore, it was concluded that there appeared the disordered 1 x 1 structure on the surface of the Si small mound, lowering the surface potential by 0.1 V. (C) 2002 Elsevier Science B.V All rights reserved.