Abstract: Provided is a crosslinked thermally rearranged poly(benzoxazole-co-imide) obtained through the transesterification crosslinking of an ortho-hydroxy polyimide copolymer and a diol compound, followed by thermal rearrangement, a gas separation membrane (excluding a membrane for flue gas separation), and a method for preparing the same. The crosslinked thermally rearranged poly(benzoxazole-co-imide) according to the present invention allows less packing of polymer chains and has a structure with a larger space, and thus shows increased possibility of permeation and diffusion of small molecules therethrough.