A new addition curable silicone resin (ASiR) system with excellent dielectric and thermal properties was developed, which consists of only two components: poly(methylphenylvinylsiloxane) (PMPVSi) and an end-capped hydrogen-functionalized hyperbranched polysiloxane (EHFHPSi). PMPVSi is synthesized by a green and controllable process; EHFHPSi is first synthesized via A2 + B3 approach, and then end-capped by hexamethyldisiloxane (HMDS). Three formulations were designed to investigate the optimum stoichiometry. Results show that cured ASiR resins have greatly different dielectric and thermal properties because of the different chemical structure of cured networks resulting from the different stoichiometries. The resin with a suitable stoichiometry has not only excellent dielectric properties including extremely low dielectric constant (2.96 at 1 Hz) and loss (0.0003 at 1 Hz) as well as good stability on frequency, but also outstanding thermal resistance, exhibiting great potential to be used as a new kind of high-performance resins for many cutting-edge industries, especially the microelectronic and insulation fields. POLYM. ENG. SCI., 2012. (C) 2011 Society of Plastics Engineers