A DGEBA epoxy resin cured using dicyandiamide hardener, was modified by using two types of CSR nano-particles over a range of volume fractions from 0 vol.% to 38 vol.%. The size and microstructure of CSR particles were studied and the measured diameters are 203 nm (with a 16.9 nm shell thickness) and 74.1 nm respectively. Addition of CSR nano particles increased the glass transition temperature due to the interaction between matrix and particles. Addition of CSR particles increased the ductility and Poisson's ratio of epoxy, but reduced the Young's modulus and tensile strength. Young's modulus, Poisson's ratio and tensile strength were well predicted by theoretical models using an effective volume fraction of rubber. An optimum CSR content was found to exist, when the fracture energy increased from 343 J/m(2) for matrix to 2671 J/m(2) for epoxy modified by 30 vol.% of CSR particles, and then started to decline. The main toughening mechanisms were proved to be debonding of CSR particles from matrix, followed by plastic voids growth, which was accompanied by shear bands yielding. Large-scale plastic void growth was discovered at the subcritical tip of double notch four points bending test (DN-4PB), and the values of fracture energy were found to be proportional to the size of plastic damage zone measured from subcritical crack tip of DN-4PB. This proved the plastic void growth is the major toughening mechanism, which correlated well with the results of fracture energy modelling. A theoretical model was used to predict the fracture toughness increment due to the shear band yielding and plastic void growth. The analytical values showed good agreement with the experimental data, and further demonstrated the proposed toughening mechanism. (C) 2015 Elsevier Ltd. All rights reserved.