Etching with molecular fluorine on a Si(111) surface was studied in an ultrahigh vacuum reactor. Reflection high-energy electron diffraction intensity oscillation during the etching was successfully observed only below 580° C. Drastic improvement of surface morphology and disappearance of the oscillation indicate an etching mode variation with increasing temperatures. An activation energy of 108 kcal/mol for the etching on flat (111) surface was estimated from the periods of the oscillations.