The relationship between etching characteristics and silicon content for organosilicon polymers has been studied. Under oxygen reactive ion etching (O2‐RIE) conditions, the etching rate for the polymers became constant after an initial rapid decrease. An etching kinetic study was carried out to account for this behavior. It is confirmed from electron spectroscopy for chemical analysis (ESCA) data that a protective layer, which has high resistance against O2‐RIE, is formed on the polymer surface and that most silicon atoms exist in the form of SiO2at the protective layer surface. The etching rate for the polymers in a steady state is inversely proportional to the silicon content. The etching rate for quartz glass is in good agreement with the value extrapolated from this relationship. These results suggest that the rate determining step in the etching process is the sputtering of SiO2formed by the polymer oxidation. Furthermore, for a polymer with a lower silicon content than a threshold value, the protective layer is porous, so that the underlying polymer is attacked by radical species during O2‐RIE. The threshold silicon content is about 0.15 g/cm3.