We have observed a twin boundary in a triblock copolymer/silica mesophase thin film, produced by spinning an acidic water-ethanol-tetraethoxysilane solution of a triblock copolymer on a hydrophilic treated silicon substrate. The highly ordered structure of the mesophase thin film is formed by ordering of self-assembled triblock copolymer micelles and was characterized with x-ray reflectivity and transmission electron microscopy. The mesophase thin film consists of a face-centered-cubic lattice and the orientation of the film surface is the {111} plane. Furthermore, a {111} twin plane is observed in the mesophase thin film. (C) 2003 American Institute of Physics.