Amine-terminated poly(arylene ether sulfone)-carboxylic-terminated butadiene-acrylonitrile-poly(arylene ether sulfone) (PES-CTBN-PES) triblock copolymers with controlled molecular weights of 15,000 (15K) or 20,000 (20K) g/mol were synthesized from amine-terminated PES oligomer and commercial CTBN rubber (CTBN 1300x13). The copolymers were utilized to modify a diglycidyl ether of bisphenol A epoxy resin by varying the loading from 5 to 40 wt%. The epoxy resins were cured with 4,4'-diaminodiphenylsulfone and subjected to tests for thermal properties, plane strain fracture toughness (K-IC), flexural properties, and solvent resistance measurements. The fracture surfaces were analyzed with SEM to elucidate the toughening mechanism. The properties of copolymer-toughened epoxy resins were compared to those of samples modified by PES/CTBN blends, PES oligomer, or CTBN. The PES-CTBN-PES copolymer (20K) showed a K-IC of 2.33 MPa m(0.5) at 40 wt% loading while maintaining good flexural properties and chemical resistance. However, the epoxy resin modified with a CTBN/8K PES blend (2:1) exhibited lower K-IC (1.82 MPa m(0.5)), lower flexural properties, and poorer thermal properties and solvent resistance compared to the 20K PES-CTBN-PES copolymer-toughened samples. The high fracture toughness with the PES-CTBN-PES copolymer is believed to be due to the ductile fracture of the continuous PES-rich phases, as well as the cavitation of the rubber-rich phases.