Three types of polyhedral oligomeric silsesquioxane (POSS) compounds were used to synthesize nanocomposites based on urethane dimethacrylate (UDMA). An advanced characterization of POSS compounds was done by X-Ray Photoelectron Spectroscopy (XPS) for chemical structure, X-Ray Diffraction (XRD) for crystalline phase and Differential Scanning Calorimetry (DSC) for thermal transitions. The POSS compounds generally influence the thermostability of UDMA-based nanocomposites acting both as a comonomer and reinforcing agent.