A study was conducted to investigate the network architecture to an epoxy system modified with a model poly(ethylene oxide)-b-poly-(ethylene-alt- propylene) (OP) block copolymer. The study demonstrated an increase in block copolymer toughening upon a decrease in the cross-link density of the epoxy matrix. Diglycydyl ether of bisphenol A based epoxy was used for the study as epoxy resin, while trifunctional cross-linker 1,1,1-tris(4-hydroxyphenyl)ethane and difunctional extender bisphenol A were used to systematically alter the network cross-link density. It was observed that the crosslink density of an epoxy network significantly influenced the efficacy of block copolymer particles in promoting fracture toughness. Macroscopic plastic deformation was visible near the crack tip for highly cross-linked systems and the deformation zone expanded with the decrease of cross-link density.