Ultrafine silicon quantum wires with high-quality Si/SiO2 heterointerfaces are successfully fabricated by utilizing anisotropic wet chemical etching and subsequent thermal oxidation. It is also found that the lateral dimensions of silicon quantum wires can be well controlled by selecting the temperature of the thermal oxidation process. The cross-sectional image from a scanning electron microscope shows silicon quantum wires of high quality with the linewidth down to 20 nn.