Methods for forming semiconductor structures are provided. The method includes forming a first fin structure and a second fin structure over a substrate and forming a first sidewall layer to cover the first fin structure and the second fin structure over the substrate. The method for manufacturing a semiconductor structure further includes forming a second sidewall layer over the first sidewall layer and etching a top portion of the first fin structure and the first sidewall layer and the second sidewall layer formed over the top portion of the first fin structure to expose a portion of the first fin structure. The method for manufacturing a semiconductor structure further includes oxidizing the exposed portion of the first fin structure to transform the exposed portion of the first fin structure into an oxide structure formed over the first fin structure.