Low dielectric polybenzoxazine nanocomposites (SBA-15/PBZ) containing mesoporous silica (SBA-15) were successfully synthesized, using benzoxazine functionalized SBA-15 (BZ/SBA-15) and benzoxazine monomer, through thermal ring opening polymerization. SBA-15 was functionalized with benzoxazine functional silane via grafting method. The benzoxazine functionality in BZ/SBA-15 was confirmed by FT-IR, C-13 CPMAS, Si-29 CPMAS NMR, XPS, TGA and nitrogen porosimetry analysis. By increasing the amount of BZ/SBA-15, the SBA-15/PBZ nanocomposites possess higher T-g and good thermal stability. 7.5 Wt % of BZ/SBA-15 reinforced polybenzoxazine nanocomposite possesses an ultra-low dielectric constant of 1.75 at 1 MHz.