Hydrolytically polymerized poly(methyl-co-vinyl)siloxane is cross-linked by radically and dehydrolytically at elevated temperature. Peroxide-type thermal radical generator is found to cross-link the polymer at the vinyl and methyl pendant groups. In parallel, free silanol (Si-OH) end groups in the polymer also contribute to cross-linking by dehydrolytic polycondensation. By use of these two-fold cross-linking mechanisms, we were able to deposit highly cross-linked siloxane polymer film which provides excellent optical and electrical properties. Cured film of 500 nm thickness exhibited the pencil hardness of 9H on glass with > 95% visible light transmittance. These excellent features are applied to optical hard coating for flexible displays and touch panels. The cured film also exhibited excellent electric properties. The leakage current of the film is as low as that of CVD dielectric film, and the break down field is exceeded 3 MV/cm, which enabled the film to be applied to insulator in thin film transistor. We carried out analyses of the polymer in film and powder form with 13C- and 29Si-NMR MAS, FT-RAMAN and FTIR-ATR methods to investigate curing mechanism. The analysis results clearly indicated that the cured film contains unique Si-(CH)n-Si bonds generated by radical crosslinking, and all the Si-OH bonds are consumed by hydrolytic polycondensation as well. The Si-(CH)n-Si bonds are more rigid and less polar than Si-O-Si bond, which should be the major reasons that radical condensation remarkably reinforced the film. This radically and thermally cured poly(methyl-co-vinyl)siloxane film was applied to Mo-gate thin film transistor fabricated on glass as the gate insulator. The I-V characteristics from the transistor were equivalent to those made using CVD-SiN insulator.