We investigated the thermal decomposition of native oxide on Si(100) under ultrahigh vacuum using high-resolution x-ray photoelectron spectroscopy (XPS). The native oxide was formed by wet chemical treatment (HCl/H2O2/H2O), a widely employed procedure for preparing atomically clean surfaces. XPS measurements revealed that high temperature heating (≳700 °C) leads to a remarkable alteration in Si 2p and O 1s spectra. After heating to 700 °C, the Si3+ structure increases and the O 1s full-width-at-half-maximum decreases. After heating to 800 °C, the Si4+ and O 1s intensity decreases but the Si2+ intensity remains almost unchanged. We suggest that the formation of volatile SiO is related to the Si3+ structure produced by high temperature annealing.