In order to improve the compatibility of organic-inorganic phases to obtain enhanced multi-physical properties in the polymer nanocomposites, surface functionalization of inorganic fillers is of great significance and has been widely employed in hybrid composite approaches. This work describes a comparative study on the effect of varied surface chemistries of nano-SiO on the electrical properties of epoxy resin nanocomposites. It is found that the electrical properties, including DC volume resistivity and dielectric breakdown strength of the nanocomposites with epoxy chain-grafted SiO are simultaneously improved in comparison with those of the neat epoxy resin as well as the composites with unmodified SiO and silane coupling agent-modified SiO. The surface-grafted epoxy chain, which has the same chemical structures with resin matrix, not only boosts the filler dispersion but also provides the active groups for the establishment of the robust bonds during the curing processes of thermoset resin composites. These results verify the structural design of organic-inorganic interface with covalent bonds is effective to enhance the electrical properties, and has general applicability and reference value for developing high-performance thermoset resin-based composites.