Two novel low dielectric polysiloxanes with high thermostability and low water uptake were developed. These polymers were formed via a thermo-crosslinking reaction of two BCB-containing organosiloxanes. They exhibited dielectric constants (D_k) of less than 2.60 for frequencies ranging from 0.15 to 30.0 MHz, and good thermostability with a 5% weight loss temperature of up to 460 °C. In particular, the polymer derived from a monomer containing three BCB units displayed a T_g of 350 °C. These polymers exhibited water uptakes of less than 0.20% when they were immersed in boiling water for 72 h. These data reflect that these polysiloxanes are suitable as high performance low dielectric resins in the microelectronics industry.