Optical microscopy in reflection and transparent modes with normal and cross-polarized lights contributes to understand the particle-aggregated structures and their peculiar characteristics in the viscosity of an epoxy resin composite system filled with silica particles. The apparent viscosity of a 17 mu m-SiO2 particle filled system shows minimum values compared with those of 13 and 27 mu m particle systems. Only SiO2 primary properties such as particle size distribution and specific surface area are not enough for the understanding of the characteristics. Optical microscopy reveals that the 17 mu m particle system has the smallest particle-aggregated structure of SiO2 in epoxy resin. This appears to explain the rheological properties and confirmed the possibility of the optical characterization to identify the particle-aggregated structure of epoxy composite filled with silica particle.