Abstract: A method for forming a device isolation film of a semiconductor device, which includes the steps of forming a pad oxide film on a semiconductor substrate, forming an oxidation buffer layer on the pad oxide film, forming an oxidation prevention film on the oxidation buffer layer, forming an aperture in the oxidation prevention film and a longitudinally co-extensive recess in the oxidation buffer layer, to thereby expose a portion of the oxidation buffer layer, forming a cap oxide film on the exposed portion of the oxidation buffer layer by subjecting a first resultant structure obtained by the preceding steps to a thermal oxidation process, forming an oxynitride film at an interface between the cap oxide film and the oxidation buffer layer by heat-treating a second resultant structure obtained by the preceding steps in a nitrogen atmosphere, and, forming the device isolation film by subjecting a third resultant structure obtained by the preceding steps to a thermal oxidation process.