Abstract: A comparative study was carried out on the rubber/epoxy composites filled with sub-micron or nano-sized rubber particles. The results show that the viscosities of uncured epoxy resins increase after adding nano-sized rubber particles, but decrease when mixed with sub-micron ones. The glass transition temperatures of the rubber/epoxy composites increase with the nano-sized rubber particles, but decrease with the sub-micron ones. All rubber particles result in decreasing the elastic modulus of the composites obviously, but increasing the elongations at break. At lower filler content, the nano-sized rubber can improve the tensile strength of epoxy. All rubber particles exhibite significant toughening effect. Based on the morphologies in micro-scale, rubber particle debonding is observed for both rubber particles, and this promotes local plastic deformation around crack tips.