Copolymerization of PBZ precursors with epoxies forms network structures with high crosslinking densities, potentially improving the thermal and mechanical properties. In this study, we copolymerized the siloxane-imide-containing benzoxazine BZ-A6 was with siloxane-epoxy GT-1000. Analyses using differential scanning calorimetry and Fourier transform infrared spectroscopy revealed that the copolymers were formed at a relatively low curing temperature of 150 °C. The siloxane benzoxazine/epoxy mixture exhibited high thermal stability, with a high char yield of 44.6% and a high decomposition temperature of 364.9 °C. Moreover, during UV exposure tests, the water contact angle of the BZ-A6/GT-1000 copolymer was more stable than that of the conventional bisphenol A-type benzoxazine-epoxy Ba/DGEBA, suggesting that our new benzoxazine/epoxy mixture would be more suitable for applications requiring hydrophobic materials that are UV resistant. The low curing temperature and good temperature- and UV-resistance of this siloxane benzoxazine/epoxy mixture should make it widely applicable, such as IC package materials (film type encapsulant, paste encapsulant) or weather resistance application.