Abstract: A method for forming device isolation film of semiconductor device is provided, the method including sequentially forming a pad oxide layer and a pad nitride layer on a semiconductor substrate having a cell region and a peripheral circuit region, etching a predetermined region of the pad nitride layer, the pad oxide layer, and the semiconductor substrate to form a trench, forming an sidewall oxide film on a surface of the trench, etching a predetermined thickness of the sidewall oxide film in the cell region, forming a liner nitride film and a liner oxide film on the semiconductor substrate including the trench and the pad nitride layer, depositing a HDP oxide film to fill up the trench, performing a planarization process to expose the pad nitride layer, and removing the pad nitride layer.