A method for fabricating shallow trench isolation structures is provided. A patterned pad layer and a patterned mask layer are sequentially formed on a substrate, wherein the substrate includes a memory region and a periphery region. By using the patterned mask layer as a mask, the substrate is partially removed to form a plurality of trenches. A first liner layer is formed on the substrate to cover surfaces of the patterned mask layer, the patterned pad layer and the trenches. After removing the first liner layer in the periphery region, a pull-back process is performed on the patterned mask layer, and a pull-back amount of the patterned mask layer in the periphery region is larger than a pull-back amount of the patterned mask layer in the memory region. An insulating layer is formed in the trenches to form a plurality of shallow trench isolation structures.