Novel nanostructured unsaturated polyester thermosets modified with two poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) block copolymers, were developed. Effects of molecular weights and blocks ratio on optical and mechanical properties were reported. It was found that self-assembly or polymerization-induced phase separation mechanisms were followed by the nanostructured thermosetting systems to achieve the final morphology. Relationships between final morphology and optical and mechanical properties of thermosets were examined.