A fabrication method of a semiconductor device is disclosed. Particularly, in the process of forming a gate oxide film on a semiconductor substrate, the method for forming a gate oxide film of a semiconductor device comprises the steps of first-annealing the semiconductor substrate in a nitrogen (N.sub.2) atmosphere; forming a gate oxide film by wet-oxidizing the annealed semiconductor substrate at a low temperature in a mixed gas atmosphere of oxygen (O.sub.2) and hydrogen (H.sub.2); and second-annealing the semiconductor substrate where gate oxide film has been formed, at a high temperature in a nitrogen (N.sub.2) atmosphere. Accordingly, the thinning phenomenon of the gate oxide film near the field oxide film is prevented and the instability such V.sub.FB in the conventional field oxidation method is considerably recovered. Also, the field concentration phenomenon is decreased and tolerance to dielectric breakdown is increased.