A method of separating semiconductor device structures comprises steps of providing a substrate having a first surface and a second surface opposite to the first surface; forming a plurality of semiconductor epitaxial stacks on the first surface; forming a patterned resist layer covering the semiconductor epitaxial stacks and exposing part of the first surface, or covering the second surface corresponding to the semiconductor epitaxial stacks; performing a physical etching process to directly server the substrate apart from an area of the first surface or the second surface not covered by the patterned resist layer; and separating the semiconductor epitaxial stacks to form a plurality of semiconductor device structures.