We have investigated a fabrication technique of nm size Si wires utilizing a bevel SiO2 wail on a (111) surface as an electron cyclotron resonance (ECR) plasma etching mask. The bevel (111) surface is obtained by anisotropic etching of a (100) silicon-on-insulator layer. Small-size Si wires of around 20-25 nm are successfully obtained underneath the bevel SiO2 wall. It is shown that a local oxidation of silicon process to form the bevel SiO2 wall affects the cross-sectional shape of the small-size Si wires in the ECR plasma etching process. It is also indicated that a self-limiting oxidation takes place in such small-size Si wires. By thermal oxidation of the small-size Si wires, an ultrasmall cross-sectional size of around 1x3 nm(2) is realized. In addition, a Coulomb blockade effect is observed in an experimental nm size Si wire metal-oxide-semiconductor field-effect transistor.