SiO2/epoxy composite was prepared with silica microspheres at different diameters as filler, which would be used in ultra high voltage DC transmission system. The effect of silica particle size on dielectric properties of the composite was studied by measuring DC volume resistivity, relative dielectric constant and dielectric loss tangent. The role of interface between silica microspheres and epoxy matrix was investigated using SEM and dynamic mechanical analysis. The results indicate that silica microspheres can significantly enhance the dielectric properties of the material. The composite with 100 nm SiO2 filler shows the best results. Its glass transition temperature and DC volume resistivity increase from 104.9℃ to 106.4℃ and 1.87′1017 Ω×cm to 3.24′1017 Ω×cm, respectively, while its relative dielectric constant decreases from 10.99 to 4.92.