A method for forming a semiconductor device is disclosed. A method for forming a semiconductor device includes forming a first hard mask layer over a semiconductor substrate including a cell region and a peripheral circuit region, forming a spacer pattern over the first hard mask layer of the cell region, forming a cell-open mask pattern over the peripheral circuit region, forming a first hard mask pattern by etching the first hard mask layer using the spacer pattern of the cell region as an etch mask, forming a second hard mask layer over the first hard mask pattern of the cell region and a first hard mask layer of the peripheral circuit region, forming a cutting mask pattern over the second hard mask layer; and forming an active region in the cell region and a device isolation region in the peripheral circuit region by etching the second hard mask layer, the first hard mask pattern of the cell region, the first hard mask layer of the peripheral circuit region, and the semiconductor substrate using the cutting mask pattern as an etch mask.