The interaction of atomic chlorine with Si(100)2×1 surfaces was studied by using chlorine atom beams. The etching reaction of silicon substrates has been observed when chlorine atoms impinged on the chlorinated surface, at substrate temperatures below 600°C. The major desorption product is SiCl2. Studies of the temperature dependence of the reaction showed that the activation energy are 0.08 eV at 0.4 ML and 0.2 eV at 0.8 ML. These extremely low activation energies suggest that the surface reaction is mainly driven by the internal energy of incident atomic chlorine instead of thermal excitation from Si(100) solid surface. Therefore chlorine atoms enter the transition state without equilibrating at the surface prior to the reaction. The reaction strongly depends on the chlorine coverage on the surface. The reaction occurred above 0.3 ML. The etching probability of the surface reached a maximum at 0.4 ML, and decreased with increasing coverage.