The double-decker-shaped silsesquioxane (DDSQ) is a new family of silsesquioxanes consisting of a Si-O-Si case structure and functional organic side chains. The optical properties of a novel organic-inorganic hybrid polymer based on the DDSQ were studied. The double-decker-shaped poly(silsesquioxane) (DDPSQ) showed excellent transparency and thermal stability. The thermo-optic coefficient of the DDPSQ is 3 or 4 times larger than conventional polymers.