A method for fabricating substrate of a semiconductor device includes the steps of: providing a first silicon layer; forming a dielectric layer on the first silicon layer; bonding a second silicon layer to the dielectric layer; removing part of the second silicon layer and part of the dielectric layer to define a first region and a second region on the first silicon layer, wherein the remaining of the second silicon layer and the dielectric layer are on the second region; and forming an epitaxial layer on the first region of the first silicon layer, wherein the epitaxial layer and the second silicon layer comprise same crystalline orientation.