Abstract In the current work, the fracture properties of epoxy polymers containing poly-siloxane core shell rubber (CSR) nano-particles were studied. The effect of different cure rates and curing temperatures on the epoxy resin was also investigated. Single edge notched bend tests were performed to evaluate the fracture energy of the polymers. The fracture energy of the unmodified epoxy polymer increased significantly from 339 J / m 2 to 3,922 J / m 2 due to the addition of 9 wt% of CSR nano-particles. Similarly, the fracture toughness for the unmodified polymer increased from 1.27 MPa m 1 / 2 to 3.42 MPa m 1 / 2 for an epoxy polymer containing 9 wt% of CSR nano-particles. Faster rates of curing, which can be achieved at higher cure temperatures are found to be detrimental to the toughness of the modified epoxy polymers.