A method for fabricating a device isolation structure of a semiconductor device includes the steps of forming a pad oxide layer and a pad nitride layer over a semiconductor substrate including a cell region and a dummy region, etching a portion of the pad nitride layer, the pad oxide layer and the semiconductor substrate to form a trench, forming a sidewall oxide layer over the sidewalls of the trench; removing the sidewall oxide layer in the dummy region, forming a silicon nitride layer over the sidewalls of the sidewall oxide layer both in the cell region and in the dummy region, filling the trench with an insulating layer, polishing the insulating layer to expose the pad nitride layer, and removing the pad nitride layer.