A semiconductor device includes a semiconductor substrate and a trench isolation. The trench isolation is located in the semiconductor substrate, and includes an epitaxial layer and a dielectric material. The epitaxial layer is in a trench of the semiconductor and is peripherally enclosed thereby, in which the epitaxial layer is formed by performing etch and epitaxy processes. The etch and epitaxy process includes etching out a portion of a sidewall of the trench and a portion of a bottom surface of the trench and forming the epitaxial layer conformal to the remaining portion of the sidewall and the remaining portion of the bottom surface. The dielectric material is peripherally enclosed by the epitaxial layer.