Disclosed herein is a method for fabricating an ultra fine nanowire, which relates to a manufacturing technology of a microelectronic semiconductor transistor. This method obtains a suspended ultra fine nanowire base on a combination of a mask blocking oxidation process and a stepwise oxidation process. A diameter of the suspended ultra fine nanowire fabricated by this method is precisely controlled to 20 nm by controlling a thickness of a deposited silicon nitride film and a time and temperature of the two oxidation process. Since a speed of a dry oxidation process is slower, the size of the final nanowire may be precisely controlled. This method can be used to fabricate an ultra fine nanowire with a lower cost and a higher applicability.