Silicon nanostructures along [011] direction with upside down triangle cross sections on the top of the sawtooth structure with (111) facets are prepared by using the lithography technique, reactive ion etching, and anisotropic wet chemical etching. These triangle‐shaped silicon nanostructures are thermally oxidized in dry oxygen over a range of temperature from 850 to 1000 °C, which is characterized by scanning electron microscopy. The self‐limiting oxidation phenomenon observed in silicon nanostructures is discussed. Cross‐sectional shape change of the silicon nanostructure under different oxidation temperatures is demonstrated. A silicon quantum wire is successfully fabricated by two‐step thermal oxidation of the silicon nanostructures.