With a view to replace the conventional insulating silica material, a thermally stable mesoporous silica/siloxane based hybrid polybenzoxazine nanocomposite was developed as an interlayer low k dielectric material. The precursors such as benzoxazine terminated polydimethylsiloxane (PDMS-Bz) and benzoxazine terminated mesoporous MCM-41 silica (BTMS) materials were synthesized and are copolymerized by a thermal curing process to obtain polybenzoxazine nanocomposites. The cured polybenzoxazine nanocomposites were characterized using different analytical techniques such as FTIR, NMR, XRD, SEM, HRTEM, DSC and TGA. The dielectric behavior of different weight percentages of BTMS reinforced PDMS polybenzoxazine (BTMS/PDMS-PBz) nanocomposites were described based on the concepts of polarization and porosity of the material. Among the different weight percentages of BTMS reinforced composites 7 wt% BTMS/PDMS-PBz possesses the lowest value of dielectric constant (2.06) with enhanced thermal stability.