The surface morphology of Si(100)-2 X 1 with submonolayers of LiF adsorbate and its annealing behavior are studied using scanning tunneling microscopy. LiF adsorbs randomly on the Si(100)-2 X 1 surface at room temperature (RT), and the 2 X 1 structure disappears when the coverage of LiF is close to 1 monolayer. Interaction of the Si surface and the LiF adsorbate is enhanced by specimen annealing, which causes dissociation of the LIF and fluorination of the Si surface. Desorption of SiFx (x = 1, 2, 3, 4) results in surface etching. After annealing at 700 degrees C for 5 min, fluorine on the surface decreased below the limit of the detection by X-ray photoelectron spectroscopy, and the Si surface is reconstructed to ?,X I at about 800 degrees C.