We report a self-modulated taper-drawing process for fabricating silica nanowires with diameters down to 20 nm. Long amorphous silica nanowires obtained with this top-down approach present extraordinary uniformities that have not been achieved by any other means. The measured sidewall roughness of the wires goes down to the intrinsic value of 0.2 nm, along with a diameter uniformity better than 0.1%. The wires also show high strength and pliability for patterning under optical microscopes. The ability to prepare and manipulate highly uniform silica nanowires map open up new opportunities for studying and using low-dimensional silica material on a nanometer scale.