Using scanning tunneling microscopy (STM), we succeeded in observing the initial process of the HBO molecule on the Si(111)-7×7 surface for the first time. Since HBO2 molecules are commonly used as the B source in the molecular beam epitaxy system, and the dominant flux from the HBO2 source in the ordinary condition is the form of HBO, analysis of the HBO reaction process is highly important. When the substrate temperature was kept at room temperature during the molecular irradiation, the adsorbed HBO molecules remained on the surface without dissociation. With the substrate temperature being increased, the dissociation process was activated, and the oxygen- and boron-related adatoms were produced at the corner and center adatom sites in the 7×7 units, respectively. The site dependence was well determined by the bias voltage dependent STM. The selective reaction site of oxygen atoms was comprehensively explained by their electronegativity and charge distribution in the Si(111)-7×7 surface adatoms. On the other hand, regarding the selectivity for the boron atoms, introduction of the surface strain effect was found to be essential. © 2000 American Vacuum Society.