A method for forming an isolation layer for a semiconductor device is provided. The preferred method is capable of securing a gap fill margin during formation of an isolation layer. A device isolation layer formed according to a preferred method includes a trench formed in a device separation area of a semiconductor substrate; a thermal oxidation layer formed in a part of the trench; an oxidation silicon layer formed on the thermal oxidation layer; and an oxidation isolation layer formed on the oxidation silicon layer and filling the trench.