New thermally curable organic/inorganic hybrid polymers were designed and synthesized as insulators for organic thin film transistors (OTFTs). Cyclotetrasiloxane (CTS) was reacted with allyl alcohols through a hydrosilylation reaction in the presence of a catalytic amount of Pt(0) to give the alcohol-functionalized cyclotetrasiloxane (CTS-OH). The synthesized CTS-OH was then thermally cured with hexamethoxymethylmelamine (HMMM) at 80 °C in the presence of a catalytic amount of p-toluenesulfonic acid to form a hard and smooth thin film composed of a highly cross-linked network polymers (CTS-MMs). Devices with indium-tin-oxide/CTS-MM/Au configuration were fabricated to investigate electrical properties of the polymers such as capacitance, dielectric constant, and leakage current. The CTS-MM showed lower leakage current level than the well-known curable insulator consisting of poly(vinylphenol) (PVP) and a melamine derivative. Pentacene-based OTFTs were fabricated using the synthesized insulators as the gate dielectric layers, and their performances were compared to those of the device fabricated using PVP. The OTFTs fabricated using CTS-MM showed higher field-effect mobility than that of the PVP. The hole mobility of the pentacene based-OTFTs fabricated using CTS-MM as gate dielectric was 0.36 cm2/V s and the on/off current ratio was >107.