The initial stage of the reaction of aluminum trichloride (AlCl3) With the Si(111)-(7X7) surface and annealing effects of the adsorbed surface are investigated with a scanning tunneling microscope. Reacted and unreacted sites manifest in the contrast of adatom sites on the AlCl3-exposed surface. An AlCl3 molecule dissociatively adsorbs onto the Si(111)-(7X7) surface at room temperature, which yields chlorine atoms. Preferential adsorption site is found to be the center adatom site rather than the comer adatom site, and the chlorine atom adsorbs onto the top site of the center Si adatom. The migration of adsorbed molecules around the rest atom in the 7X7 unit cell is observed at room temperature. The Si-Cl(x) species desorb from the surface by thermal annealing at 600-degrees-C, leaving vacancies behind, after annealing at 1200-degrees-C. Aluminum deposition occurs in a limited area and the surface shows the square-root 7 X square-root 7 reconstruction.