The core-shell particles considered were poly(butyl acrylate) core/epoxy groups functionalizing the poly(methyl methacrylate) shell. Physical and thermomechanical properties of benzyl dimethylamine (BDMA)-catalyzed diglycidyl ether of bisphenol A (DGEBA)/dicyandiamine epoxy networks toughened with core-shell particles were studied. The blends were prepared under well-defined processing conditions. The resulting properties were found to depend on the state of the dispersion of the particles in the prepolymer matrix before crosslinking. These particles were dispersed at different volume fractions in order to vary the interparticle distance. The relationships between the size of the core-shell particles and the level of toughening are reported. Static mechanical tests were performed in tension and compression modes on these core-shell polyepoxy blends. A slight decrease in the Young's modulus and an increase in the ability to plastic deformation were observed. Using linear fracture mechanics (LEFM), an improvement of the fracture properties (K-IC) was measured. By varying the volume fraction of core-shell particles, an optimum toughness improvement was found for an interparticle distance equal to 400 nm (with an average particle size of 600 nm).