The present disclosure provides a gas separation membrane using a ladder-structured polysilsesquioxane in which organic functional groups are attached to a long siloxane chain and a method for fabricating the same. In accordance with the present disclosure, a free-standing ladder-structured polysilsesquioxane-based membrane with various functional groups can be fabricated for gas separations. The performance of the gas separation membrane can be controlled through a variety of combinations of the organic functional groups linked to the siloxane chain. In addition, by controlling the molecular structure of the gas separation membrane through mixing of the ladder-structured polysilsesquioxane with an amine compound or conventional glassy or rubbery polymers or through thermal/UV-curing, the permeability and selectivity of the gas separation membrane can be controlled selectively.