The contribution of particle structure to the viscosity of a SiO2-filled epoxy resin system could be revealed as one of the intrinsic properties of SiO2 by using transparent optical microscopy with normal/cross-polarized lights. This was found to be a reliable tool for identifying particle-aggregated structures and their peculiar viscosity characteristics. The SiO2-filled epoxy resin system contained elliptical-shaped features under transparent microscopy in normal light, which were observed as brightened domains under cross-polarization. The elliptical features exhibited repeated bright and dark changes for rotating specimens at every 45 degrees increment. The origin of the elliptical brightness was induced from the particle-aggregated structures. The elliptical brightness i.e. optical anisotropic property, was often observed in SiO2 particle systems with smaller amounts of fine particles on the surface of the SiO2 filler core particles. The optical anisotropy explained the deconstruction/reconstruction process of particle agglomeration and viscosity in changing the particulate composite structures. However, their theological properties remained unclear solely from the well-mentioned filler primary properties based on the characterization of the SiO2-filled epoxy resin system: median diameters, size distributions, specific surface areas and surface hydroxyl group structures.