The invention provides a semiconductor structure, the semiconductor structure includes a substrate, two shallow trench isolation structures are located in the substrate, a first region, a second region and a third region are defined between the two shallow trench isolation structures, the second region is located between the first region and the third region. Two thick oxide layers are respectively located in the first region and the third region and directly contact the two shallow trench isolation structures respectively, and a thin oxide layer is located in the second region, the thickness of the thick oxide layer in the first region is greater than that of the thin oxide layer in the second region.