A method for forming a device isolation layer of a semiconductor device or a non-volatile memory device is provided. A method for forming a device isolation layer of a semiconductor device includes: forming trenches having a first predetermined depth by etching a substrate; forming a first insulation layer having a second predetermined depth inside the trenches; forming a liner oxide layer having a predetermined thickness on internal walls of the trenches with the first insulation layer formed therein; and forming a second insulation layer for forming a device isolation layer over the substrate with the liner oxide layer formed therein, wherein the second insulation layer has a lower etch rate than that of the first insulation layer.