An exemplary method of etching an oxide layer and a nitride layer is provided. In particular, a substrate is provided. A surface of the substrate has an isolating structure projecting therefrom. A first oxide layer, a nitride layer and a second oxide layer are sequentially provided on the surface of the substrate, wherein the first oxide layer is uncovered on the isolating structure, the nitride layer is formed overlying the first oxide layer, and the second oxide layer is formed overlying the nitride layer. An isotropic etching process is performed by using an etching mask unmasking the isolating structure, and thereby removing the unmasked portion of the second oxide layer and the unmasked portion of the nitride layer and further exposing sidewalls of the isolating structure. The unmasked portion of the first oxide layer generally is partially removed due to over-etching.