The cross-sectional Structure of Si (5 5 1 2) reconstructed surface was observed by ultrahigh vacuum high resolution transmission electron microscopy (UHV-HRTEM) in the form of a profile view method. Image simulation results using probable Structure models were calculated and compared with the experimental HRTEM images. It was found that none of the models including pi-chains explains the present H RTEM images. A new model composed of adatoms and dimers was constructed so that its simulated image agrees with the HRTEM image. (C) 2002 Elsevier Science B.V. All rights reserved.