The fracture toughness of a series of ring-opening metathesis polymerization-based Dilulin/dicyclopentadiene (DCPD) copolymers was evaluated by utilizing the essential work of fracture method and the structure-fracture property relationship was thoroughly analyzed. Both dynamic mechanical analyzer and scanning electron microscope demonstrated a reaction-induced phase separation in the copolymers. The copolymers' composition-dependent morphologies showed a significant effect on their fracture behavior. The maximum value for the essential work of fracture was found with Dil30DCPD70, which may be explained by their less heterogeneous structure and high cross-link density. The highest non-essential work of fracture was observed with samples made from Dil40DCPD60, which is possibly a consequence of the formation of a rigid DCPD-phase.