A block-copolymer of the type A-B-A was synthesized from a commercial polybutadiene rubber (PBD) and an unsaturated polyester (UP) prepolymer. The reaction process was monitored via Fourier transform-infrared spectroscopy. The obtained copolymer was used as compatibilizing agent for UP/PBD blends. Fracture measurements performed on the cured materials demonstrated a substantial enhancement of tougheness in the presence of the copolymer. The morphological analysis of the fracture surfaces showed that the presence of the compatibilizing agent produced a strong reduction of the site of the rubber particles, a better interface adhesion and an increase of localized shear yielding in the UP matrix around the rubber particles.