Disclosed herein is a method for fabricating an ultra-fine nanowire by combining a trimming process and a mask blocking oxidation process. The ultra-thin nanowire is fabricated by a combination of performing a trimming process on a mask to reduce a width of the mask and blocking an oxidation through the mask. A diameter of the floated ultra-thin nanowire fabricated by the method is controlled to 20 nm below by a thickness of a deposited silicon oxide film, a width of the silicon oxide nanowire after trimming, and a time and a temperature for performing a wet oxidation process. Also, since a speed of the wet oxidation process is faster, the width of the nanowire obtained by a conventional photolithography is reduced faster. Moreover, when fabricating an ultra-thin nanowire by using the method, the cost is reduced and it is more feasible to be implemented.