The invention provides a method for forming a semiconductor structure, which comprises providing a substrate, sequentially a first groove and a second groove are formed in the substrate, the depth of the first groove is different from the depth of the second groove, a first oxide layer is formed in the first groove, a second oxide layer is formed in the second groove, an etching step is performed to remove part of the first oxide layer, a first gate structure is formed on the first oxide layer, and a second gate structure is formed on the second oxide layer.