The ability to control structural dimensions below 5 nm is essential for a systematic study of the optical and electrical properties of Si nanostructures. A combination of electron beam lithography, NF3 reactive ion etching, and dry thermal oxidation has been successfully implemented to yield 2-nm-wide Si nanowires with aspect ratio of more than 100 to 1. With a sideview transmission electron microscopy technique, the oxidation progression of Si nanowires was characterized over a range of temperature from 800 to 1200°C. A previously reported self-limiting oxidation phenomenon was found to occur only for oxidation temperatures below 950°C. A preliminary model suggests that increase in the activation energy of oxidant diffusivity in a highly stressed oxide may be the main mechanism for slowing down the oxidation rate in the self-limiting regime.