Embodiments relate to a method for fabricating a semiconductor device. In embodiments, the method may include forming a gate dielectric layer on an active region of a semiconductor substrate defined by an isolation region to form a gate conductive layer pattern, etching the isolation region of the semiconductor substrate where the gate conductive layer pattern is formed, to form an isolation trench, forming a polyoxide layer on the gate conductive layer pattern and a sidewall oxide layer in the trench by carrying out an oxidation process, forming a spacer nitride layer on the polyoxide layer and a liner nitride layer on the sidewall oxide layer by carrying out a nitride layer forming process, and then forming a dielectric layer on an entire surface of the resultant structure to fill the trench.