A manufacturing method for a shallow trench isolation. First, a substrate is provided, a hard mask layer and a patterned photoresist layer are sequentially formed on the substrate, at least one trench is then formed in the substrate through an etching process, the hard mask layer is removed. Afterwards, a filler is formed at least in the trench and a planarization process is then performed on the filler. Since the planarization process is performed only on the filler, so the dishing phenomenon can effectively be avoided.