A semiconductor structure and a method for forming the same are provided. The semiconductor structure includes a gate structure, a gate spacer and a source/drain structure. The gate structure is positioned over a fin structure. The gate spacer is positioned over the fin structure and on a sidewall surface of the gate structure. The source/drain structure is positioned in the fin structure and adjacent to the gate spacer. The source/drain structure includes a first source/drain epitaxial layer and a second source/drain epitaxial layer. The first source/drain epitaxial layer is in contact with the fin structure. The first source/drain epitaxial layer is connected to a portion of the second source/drain epitaxial layer below a top surface of the fin structure. The lattice constant of the first source/drain epitaxial layer is different from the lattice constant of the second source/drain epitaxial layer.