The invention provides a method for fabricating a deep trench isolation including: providing a substrate; forming a first trench in the substrate; conformally forming a first liner layer on the sidewall and bottom of the first trench; forming a first filler layer on the first liner layer and filling the first trench; forming an epitaxial layer on the substrate and the first trench; forming a second trench through the epitaxial layer and over the first trench; conformally forming a second liner layer on the sidewall and bottom of the second trench; and forming a second filler layer on the second liner layer and filling the second trench.