Provided is a method of manufacturing a shallow trench isolation (STI) film without voids or added processes. In one embodiment, the method of manufacturing an STI film includes forming a pad oxide pattern film and a silicon nitride film pattern, which define an isolation region, on a semiconductor substrate, and forming a trench by etching the semiconductor substrate to a predetermined depth using the pad oxide film pattern and the silicon nitride film pattern as masks. The resultant semiconductor substrate having the trench may be then dipped in a chemical solution containing ozone to pullback side walls of the silicon nitride film pattern. Afterward, the STI film can be formed by filling the trench with an insulating film.