Fracture toughness of rubber modified epoxy systems was evaluated in relation to stress-whitening. The epoxy systems consisted of diglycidyl ethers of bisphenol A (DGEBA)based epoxy resin, 4,4’ diaminodiphenyl sulphone (DDS) as curing agent, and carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber. It was found that a peak value of fracture toughness occurs at a small amount of rubber content (similar to 4 phr) and closely corresponds to that of stress-whitening size. Other properties such as flexural strength and flexural modulus were also found to display maxima at a similar amount of rubber content.