This article reports the fabrication of sub‐10‐nm Si lines bye‐beam lithography and aqueous potassium hydroxide (KOH)‐based etching. The fabrication technique provides both the high resolution and minimum linewidth fluctuation necessary for fabricating nano‐scale Si structures. Ine‐beam lithography, the combination of high‐resolution posi‐type resist and hexyl acetate developer is effective in improving the resolution and in reducing pattern fluctuation. Resist lines less than 20 nm wide with a fluctuation less than 3 nm are obtained. When these lines, aligned in the 〈112〉 direction on a (110) Si wafer, are transferred to Si by KOH etching, Si lines with a rectangular cross section are obtained and linewidth fluctuation is reduced further because (111) planes are exposed as the sidewalls. In addition, 6–9 nm wide lines can be formed by additional etching using an alcohol/KOH solution. Photoluminescence spectra are obtained for sub‐10‐nm Si lines after slight oxidation.