A method for forming epitaxial layer is disclosed. The method includes the steps of providing a semiconductor substrate, and forming an undoped first epitaxial layer in the semiconductor substrate. Preferably, the semiconductor substrate includes at least a recess, the undoped first epitaxial layer has a lattice constant, a bottom thickness, and a side thickness, in which the lattice constant is different from a lattice constant of the semiconductor substrate and the bottom thickness is substantially larger than or equal to the side thickness.