Grafted triglycerides prepared from an epoxidized triglyceride and an acid anhydride containing 4 to 40 carbon atoms are reacted with an epoxy resin and an anhydride curing agent to yield an epoxy thermoset. By varying the length of fatty residues on the grafted triglyceride, the number of fatty residues per triglyceride, the identity of the epoxy resin and the anhydride curing agent, it is possible to prepare epoxy thermosets that exhibit superior physical properties compared to the properties of epoxy thermosets prepared without the grafted triglyceride, or as compared to epoxy thermosets wherein the epoxidized triglyceride is used in place of the grafted triglyceride. A significant improvement in fracture toughness with a minimal reduction of glass transition temperature may be thus achieved.