Polyhedral oligomeric silsesquioxane (POSS)-based photocurable insulating materials with methacryl (POSS-MA) or methylmethacryl (POSS-MMA) functional groups were used as gate dielectrics for pentacene-based organic thin-film transistors (OTFTs). Thin films of POSS-MA and POSS-MMA were cross-linked and completely solidified under UV irradiation in the presence of selected photoradical initiators. Metal/insulator/metal devices with a structure of indium tin oxide (ITO)/insulator/Au were fabricated to measure the leakage current and capacitance of the POSS-MA and POSS-MMA thin films. Pentacene-based OTFTs were fabricated using the synthesized POSS derivatives as gate dielectric layers, and the performance of the devices was compared with that of an OTFT fabricated using a well-known poly(vinylphenol) (PVP) insulator. The performance of the OTFTs fabricated using the POSS-MA and POSS-MMA film was comparable to that of the device fabricated using the PVP insulator. The highest mobility of the pentacene-based OTFTs using POSS-MA and POSS-MMA insulators were 0.13 and 0.17 cm2/V s, respectively.