This work investigates the role of different concentrations of a polyester‐polyurethane‐based block copolymer (BCP) on epoxy resin nanocomposites with allotropic carbon nanoparticles (graphene nanoplatelets [GNPs] or carbon nanotubes [CNTs]) on thermal, morphological, and mechanical properties, focusing on fracture toughness and its mechanisms. Amounts of BCP corresponding to 1x, 5x, and 10x the mass of nanoparticles were added. Nanocomposites with a low concentration of BCP (1x) were stronger than the neat epoxy, while high concentrations of BCP (10x) yielded a negative effect for both studied carbon nanoparticles. Fracture toughness (KIc) performed better at a 5x BCP concentration and this effect was greater for the system with CNTs. Likewise, for the GIc, the best results were seen in nanocomposites with 5x BCP, with a greater increase in the system with GNPs. In all nanocomposite systems with BCPs, the soft phase self‐arrangement mechanisms, that is, debonding/cavitation and consequent voids on the fracture surface, were verified.