An embodiment of the invention relates to a method of forming an isolation layer of a flash memory device. An isolation layer is formed using a PSZ-based material and a nitride film of liner form is deposited on a trench before the PSZ film is deposited. An oxide film can be prevented from remaining on a top of the sidewalls of a conductive film for a floating gate through an etch process employing the etch rate. The thickness of a dielectric film can be prevented from increasing when a dielectric film is deposited. Accordingly, the contact area of the floating gate and the dielectric film can be increased and the coupling ratio between the floating gate and the control gate can be improved.