Provided is a method of fabricating a semiconductor device, including the following. A first material layer, a second material layer and a mask layer are formed on a substrate. A portion of the second material layer is removed by performing a first etching process with the mask layer as a mask, so as to expose the first material layer and form a first pattern layer and a second pattern layer. A portion of the first material layer is removed by performing a second etching process with the mask layer as a mask, so as to expose a portion of the substrate. A portion of the substrate is removed by performing a third etching process with the mask layer as a mask, so as to form first trenches and second trenches. Sidewalls of the second trenches and a surface of the substrate form at least two different angles.