In this study, the carbon/epoxy composites were fabricated by adding silica nanoparticles to the epoxy resin, and the effects of silica addition on epoxy resin and composites materials were investigated. As silica content increased, the index of thermal stability and integral procedural decomposition temperature of epoxy-silica mixture increased, which means that the thermal stability improved with silica content. When the silica content was 20 wt%, the composites had the highest crosslinking density and storage modulus. The addition of silica also improved the tensile strength of composites, but excess addition of silica reduced the tensile strength and tensile modulus due to agglomeration of silica particles in the epoxy resin. Furthermore, when the carbon/epoxy composites was heat-treated at a temperatures above the glass transition temperature and the thermal decomposition temperature of epoxy, and the mechanical properties of composites were decreased due to phase transition and decomposition of epoxy resin.