The blending method was used to prepare nano-SiO2/epoxy composites, where nano-SiO2 was premodified using silane coupling agent. The influences of nano-SiO2 content on the microstructure, thermal stability and dielectric properties of SiO2/EP composites were studied by using IR, SEM, TGA, impedance analyzer et al. The microscopic mechanism of thermal stability and dielectric property evolutions of the SiO2/EP composites were also discussed. with the increase of nano-SiO2 content, the thermal stability of SiO2/EP composites gradually increased, while the dielectric constant and dielectric loss factor of the composites decreased first and then increased. When the content of nano-SiO2 equaled 4%, the nanoparticles exhibited a uniform dispersion in composite, its dielectric constant was 2.86, and its corresponding dielectric loss was 0.023 53.